A composition suitable for use in bonding synthetic polymeric fibrous materials to elastomers comprises an aqueous medium which contains a finely divided dimer or polymer of a polyisocyanate having at least one uretidinedione ring in the molecule and a latex of natural rubber or a synthetic elastomer. The dimers or polymers of polyisocyanates which contain uretidinedione rings may be based on monomeric organic compounds containing at least two isocyanate groups in the molecule at least one of which must be capable of undergoing di- or polymerization with another molecule of the polyisocyanate so that a uretidinedione ring forms, e.g. toluylene-2:4-diisocyanate, toluylene - 2:4:6 - triisocyanate, diphenylmethane - 4:41 - diisocyanate and polyisocyanates obtained by polymerization of vinyl isocyanate alone or with other vinyl compounds, by splitting off hydrogen chloride from halogeno-methyl-aryl isocyanates by means of Friedel-Crafts catalysts or by phosgenation of polymeric compounds containing primary amino groups. The dimers or polymers of the polyisocyanate groups whose reactivity has been lowered by steric or mesomeric influence or by electrostatic induction. The composition may also contain a compound having a phenolic hydroxyl group and formaldehyde or a condensate thereof or a condensate of an aldehyde with urea, melamine, a ketone or casein, anionic, cationic or non-ionic wetting agents, water-soluble protective colloids, e.g. cellulose methyl ether, polyvinyl methyl ether, polyethylene glycols, polyvinyl alcohols which may be partially acetylated polyvinyl pyrrolidone, salts of polyacrylic acids or alginic acids, polyacrylamide, proteinic substances or partially desulphurized lignosulphonates, catalysts, e.g. triethyl phosphine, triethyl amine, pyridine or its homologues, zinc adipate, titanium oxychloride or di-n-butyl tin oxide, antioxidants, pigments or dyestuffs and may be adjusted to pH in the acid or basic range by the addition of acetic or hydrochloric acid, ammonia, an alkali metal hydroxide, mono-di or triethanolaminic or am quaternary ammonium base. The latex may be of natural rubber, a terpolymer of vinyl pyridine, butadiene and styrene, hevea rubber, polybutadiene, polyisoprene, butyl rubber, copolymers of butadiene and styrene or acrylonitrile or of ethylene and propylene, polychlorobutadiene, polymeric sulphochlorinated ethylene, fluorine-containing elastomers, silicone rubbers, polysulphide rubbers, cross-linked polyurethanes, polyvinyl chloride, polyvinylidene chloride or polyvinyl acetate.ALSO:The adhesion of synthetic polymeric fibrous materials, e.g polyester fibres, filaments, threads, cables, cords and woven, knitted, braided and non woven fabrics, to elastomers is improved by treating them with a finely divided dimer or polymer of a polyisocyanate having at least one uretidinedione ring in the molecule and a latex of natural rubber or a synthetic elastomer, the treatment being carried out in one or more aqueous baths and drying them. The dimers or polymers of polyisocyanates which contain uretidinedione rings may be based on monomeric organic compounds containing at least two isocyanate groups in the molecule at least one of which must be capable of undergoing di- or polymerisation with another molecule of the polyisocyanate so that a uretidinedione ring forms e.g. toluylene -2:4-diisocyanate, toluylene-2:4:6-triisocyanate, diphenylmethane-4:4\sv-diisocyanate and polyisocyanates obtained by polymerisation of vinyl isocyanate alone or with other vinyl compounds, by splitting off hydrogen chloride from halogeno-methyl-aryl isocyanates by means of Friedel-Crafts' catalysts or by phosgenation of polymeric compounds containing primary amino groups. The dimers or polymers of the polyisocyanates may also contain one or more isocyanate groups whose reactivity has been lowered by steric or mesomeric influence or by electrostatic induction. The treatment bath or baths may also contain a compound having a phenolic hydroxyl group and formaldehyde or a condensate thereof or a condensate of an aldehyde with urea, melamine, a ketone or casein, anionic, cationic or non-ionic wetting agents, water-soluble protective colloids, e.g. cellulose methyl ether, polyvinyl methyl ether, polyethylene glycols, polyvinyl alcohols which may be partially acetylated, polyvinyl pyrrolidone, salts of polyacrylic acids or alginic acids, polyacrylamide, proteinic substances or partially desulphurized lignosulphonates, catalysts, e.g. triethyl phosphine, triethyl amine, pyridine or its homologues, zinc adipate, titanium oxychloride or di-n-butyl tin oxide, antioxidants, pigments or dyestuffs and may be adjusted to a pH in the acid or basic range by the addition of acetic or hydrochloric acid, ammonia, an alkali metal hydroxide, mono-, di- or triethanolamine or a quarternary ammonium base. When the dimer or polymer of the polyisocyanate and the latex are applied separately the polymer of the isocyanate may be applied with or immediately after the spinning preparation, after twisting, before stretching, between the first and second stretching operations, after stretching or after heat-setting and the latex applied subsequently after drying, e.g. after heat-setting or after the filaments have been made up into yarn, cord or fabric. The latex may be of natural rubber, a terpolymer of vinyl pyridine, butadiene and styrene, hevea rubber, polybutadiene, polyisoprene, butyl rubber, copolymers of butadiene and styrene or acrylonitrile or of ethylene and propylene, polychlorobutadiene, polymeric sulphochlorinated ethylene, fluorine-containing elastomers, silicone rubbers, polysulphide rubbers, cross-linked polyurethanes, polyvinyl chloride, polyvinylidene chloride or polyvinyl acetate.ALSO:The adhesion of fabrics made from synthetic fibres e.g polyester fibres, to elastomers is improved by treating them with a finely divided dimer or polymer of a polyisocyanate having at least one uretidinedione ring in the molecule and a latex of natural rubber or a synthetic elastomer, the treatment being carried out in one or more aqueous baths and drying them. The dimers or polymers of polyisocyanates which contain uretidinedione rings may be based on monomeric organic compounds containing at least two isocyanate groups in the molecule at least one of which must be capable of undergoing di- or polymerisation with another molecule of the polyisocyanate so that a uretidinedione ring forms e.g. toluylene -2:4-diisocyanate, toluylene-2:4:6-triisocyanate, diphenylmethane-4:4'-diisocyanate and polyisocyanates obtained by polymerisation of vinyl isocyanate alone or with other vinyl compounds, by splitting off hydrogen chloride from halogeno-methyl-aryl isocyanates by means of Friedel-Crafts' catalysts or by phosgenation of polymeric compounds containing primary amino groups. The dimers or polymers of the polyisocyanates may also contain one or more isocyanate groups whose reactivity has been lowered by steric or mesomeric influence or by electrostatic induction. The treatment bath or baths may also contain a compound having a phenolic hydroxyl group and formaldehyde or a condensate thereof or a condensate of an aldehyde with urea, melamine, a ketone or casein, anionic, cationic or non-ionic wetting agents, water-soluble protective colloids, e.g. cellulose methyl ether, polyvinyl methyl ether, polyethylene glycols, polyvinyl alcohols which may be partially acetylated, polyvinyl pyrrolidone, salts of polyacrylic acids or alginic acids, polyacrylamide, proteinic substances or partially desulphurized lignosulphonates, catalysts, e.g. triethyl phosphine, triethyl amine, pyridine or its homologues, zinc adipate, titanium oxychloride or di-n-butyl tin oxide, antioxidants, pigments or dyestuffs and may be adjusted to a pH in the acid or basic range by the addition of acetic or hydrochloric acid, ammonia, an alkali metal hydroxide, mono-, di- or triethanolamine or a quarternary ammonium base. The latex may be of natural rubber, a terpolymer of vinyl pyridine, butadiene and styrene, hela rubber, polybutadiene, polyisoprene, butyl rubber, copolymers of butadiene and styrene or acrylonitrile or of ethylene and propylene, polychlorobutadiene, polymeric sulphochlorinated ethylene, fluorine-containing elastomers, silicone rubbers, polysulphide rubbers, cross-linked polyurethanes, polyvinyl chloride, polyvinylidene chloride or polyvinyl acetate.