Chlorine-containing polymers of one or more monomers containing a single CH2 = C< group are plasticized by an ester of the general formula <FORM:0884660/IV (a)/1> wherein the R groups, which may be the same or different, represent hydrocarbon groups which are free of aliphatic and cycloaliphatic unsaturation and are such that the RCO- groups each contain 5-7, or an average of 5 to about 7,5 carbon atoms and x represents an integer from 1 to 3, the ester having a melting point not exceeding about 35 DEG C., and having an H number in the range of 8 to 16,5 or an average H number in the range 8 to 25,5, or are plasticized by a mixture of esters, each of the above general formula wherein the R groups represent the same or different hydrocarbon groups free from aliphatic and cycloaliphatic unsaturation and x represents an integer from 1 to 14, the mixture of esters being such that its melting point is not above about 35 DEG C., the H number is from 8 to 16,5 or the average H number is from 8 to 25,5, the RCO- groups therein each contain 5 to 7, or an average of 5 to 7,5, carbon atoms and the value of x in each ester molecule is an integer of 1 to 3 or the average value of x in the mixture is from 1 to 3. The H number of a carboxylic acid is computed by summing prescribed values allotted to the number and character of the carbon atoms a -, b - and g - to the carboxylic group and to the number and position of any aromatic rings present. H numbers are given for numerous carboxylic acids whose acyl groups may be present singly or in admixture in the esters or ester mixtures, including straight and branched chain alkanoic, cycloalkanoic, cycloalkylalkanoic, alkylcycloalkanoic, aromatic, aralkanoic and alkylaromatic acids; typical acids and their H numbers are acetic 0, benzoic 1, n-hexanoic 65, a -methylbutyric 10, triphenylacetic 13,5 and tri(t-butyl)acetic 37,5. Many examples are given of simple, mixed and mixtures of esters including dipentaerythritol hexa (a -ethylbutyrate), pentaerythritol tetra(a -ethyl-methylbutyrate) and tripentaerythritol tetra(diisopropylacetate)-tri(acetate)-myristate. The esters preferably have a purity corresponding to an acid number below 2 and a hydroxyl number below 15. Specified polymers are polyvinyl chloride and copolymers of vinyl chloride with vinylidene chloride or a vinyl ester of a lower aliphatic acid such as acetic or propionic acid. The esters may be used with other plasticizers such as a phthalate ester or a conventional dipentaerythritol ester. There may also be present anti-oxidants such as phosphites, amines and phenols, pigments, clay fillers, lubricants and polymer stabilizers such as complex lead silicatelead sulphate salt. The plasticized polymers may be used as electrical insulators of improved stability. The preparation of the esters is described (see Group IV(b)).ALSO:The invention comprises esters of the general formula <FORM:0884660/IV (b)/1> wherein the R groups, which may be the same or different, represent hydrocarbon groups which are free of aliphatic and cycloaliphatic unsaturation and are such that the RCO-groups each contain 5-7, or an average of 5 to about 7.5, carbon atoms and x represents an integer from 1 to 3, the ester having a melting point not exceeding about 35 DEG C., and having an H number in the range 8 to 16.5 or an average H number in the range 8 to 25.5. The invention also comprises a mixture of esters each of the above general formula wherein the R groups represent the same or different hydrocarbon groups free from aliphatic and cycloaliphatic unsaturation and x represents an integer from 1 to 14, the mixture of esters being such that its melting point is not above about 35 DEG C., the H number is from 8 to 16.5 or the average H number is from 8 to 25.5, the RCO-groups therein each contain 5 to 7, or an average of 5 to 7.5, carbon atoms and the value of x in each ester molecule is an integer of 1 to 3 or the average value of x in the mixture is from 1 to 3. The H number of a carboxylic acid is computed by summing prescribed values allotted to the number and character of the carbon atoms a -, b - and g - to the carboxylic group and to the number and position of any aromatic rings present. H numbers are given for numerous carboxylic acids whose acyl groups may be present singly or in admixture in the esters or ester mixtures, including straight and branched chain alkanoic, cycloalkanoic, cycloalkylalkanoic, alkylcycloalkanoic, aromatic, aralkanoic and alkylaromatic acids; typical acids and their H numbers are acetic 0, benzoic 1, n-hexanoic 6.5, a -methylbutyric 10, triphenylacetic 13.5 and tri (t-butyl) acetic 37.5. Many examples are given of simple, mixed and mixtures of esters including dipentaerythritol hexa (a -ethylbutyrate), pentaerythritol tetra (a -ethylmethylbutyrate) and tripentaerythritol tetra (diisopropylacetate)-tri(acetate)-myristate. The esters or ester mixtures may be prepared by esterifying an appropriate polyol or mixture of polyols with an appropriate carboxylic acid or mixture of acids or with the anhydrides thereof. Suitably, the reaction is effected at about 125 DEG C. to 250 DEG C., under conditions favouring water removal in the presence of an esterification catalyst and if desired a solvent, using an excess of the carboxylic acid. Many suitable catalysts are specified. Alternatively, a polyol esterified with lower aliphatic acids may be heated under transesterification conditions with the required amount of appropriate acid or lower alkyl ester thereof until the desired ester is formed. The product is recovered by stripping off unchanged reagent and/or solvent under reduced pressure and may then be treated with carbon black. A product of minimum hydroxyl number may be obtained by treatment of the crude ester with e.g., acetic, propionic or butyric anhydride, acetyl chloride or a ketene and a product of minimum acidity by alkaline washing of the stripped ester. The esters may be used as plasticizers for chlorine-containing vinyl-type polymers (see Group IV(a)). Comparative experiments describe the preparation of various hexa-esters of dipentaerythritol not having the above specified characteristics and these esters are incompatible with polyvinyl chloride.