A mixture of oxide-forming volatile metal and/or metalloid compounds is homogeneously mixed with a free oxygen-containing gas and a combustible gas and the mixture fed to a burner and reacted in the flame to form mixed or combined or partially combined oxides suitable for use as a filler for natural and synthetic rubber and other elastomers. The proportions of the constituents of the homogeneous mixture are preferably stoichiometric for the formation of the oxides and complete combustion of the combustible gas. A suitable combustible gas is carbon monoxide or, to produce a more rapid reaction and finer particles, hydrogen or gas mixtures or gases which contain or yield hydrogen (e.g. methane or illuminating gas) and thus result in the production of water in situ. In order to reduce the particle size the dilution of the oxide-forming compounds may be increased, e.g. by increasing the proportion of oxygen-containing gas such as air and, to avoid reduction in throughput, increasing the input velocity. It is preferred to use as low a flame temperature as possible, e.g. 900 to 1200 DEG C., to avoid healing and eliminating lattice faults in the oxides, by regulating the amount of combustible gas or by supplying an inert gas such as nitrogen to the flame. The latter expedient also reduces the particle size. Similarly a short time of stay in the flame is desirable and this may be achieved by the use of large but short flames, e.g. as produced by a sprinkler type of burner illustrated in Specification 726,250. The mixture is preferably supplied to the flame in laminer flow (as described in the above Specification) to promote homogeneous distribution therein, and to avoid deposition on the rim of the burner, washing gases such as air or hydrogen, or mechanical stripping may be employed. To promote the formation of a favourable product, the aerosol resulting from the reaction is preferably allowed to condense to an aerogel in the presence of previously coagulated particles, and to this end a long path of travel and vigorous movement of the aerosol is desirable, e.g. by whirling in large chambers or movement through long tubes with several bends to keep the particles in suspension until coagulation takes place, e.g. for 3, preferably 5 seconds, before separation in cyclones. Specified volatile compounds are the halides, especially chlorides, of silicon, aluminium, titanium, zirconium, iron and chromium; organometallic compounds such as aluminium ethyl, silicon tetra-ethyl, silicon tetramethyl and trichlorosilane; mixtures referred to are silicon tetrachloride and aluminium chloride (to form a substance which is the preliminary stage of an aluminium silicate), the aluminium chloride preferably being present as 0.5 to 30 per cent by weight of the mixture, silicon and titanium tetrachlorides, the latter being present to the extent of 5 to 35 per cent by weight of the mixture; fillers consisting of titanium and aluminium oxides, zirconium and aluminium oxides, or said oxides with iron, chromium or vanadium oxides are mentioned.ALSO:Rubber is made up into a mix with a filler produced by homogeneously mixing a mixture of oxide-forming volatile metal and/or metalloid compounds with a free oxygen-containing gas and a combustible gas, feeding the homogeneous mixture to a burner and reacting the mixture in a flame (see Group III). The products, which may be mixed or combined or partially combined oxides are preferably in the form of aerogels and may have been obtained from the halides (silicon, aluminium, titanium, zirconium, iron and chromium chlorides) or organometallic compounds (aluminium ethyl, silicon tetra-ethyl, silicon tetra-methyl and trichlorosilane) of the metal or metalloids, and and specific mixtures referred to are silicon tetrachloride and aluminium chloride, (to form a substance which is the preliminary stage of an aluminium silicate) and silicon and titanium tetrachlorides; fillers consisting of titanium and aluminium oxides, zirconium and aluminium oxides or said oxides with iron, chromium or vanadium oxides are mentioned. In examples, (1) 40 parts of 10 per cent alumina, 90 per cent silica filler and 100 parts of rubber were vulcanized and (2) a mixture as in (1) and containing also stearic acid (1 part) Ozokerite (2,7 parts) zinc oxide (5 parts) "Vulcazit" (Registered Trade Mark) accelerator (2,5 parts) and sulphur (4,0 parts) was vulcanized. Specification 726,250 [Group III] is referred to.