Mould fouling of EPDM rubber compounds has been studied by means of Fourier IR spectroscopy, X-ray diffractometry, differential scanning calorimetry and other methods. Chemically altered constituents of the plasticizer oil - resinous substances formed by reaction with the curing system during heating in contact with the metal surface of the mould - are the source of the soluble organic part of mould fouling. The insoluble, inorganic part is ZnS which forms when sulphur-containing vulcanizing systems are used. Vulcanization accelerators and other compounding ingredients migrate to the interface between the rubber compound and the metal of the mould, entrained on the shoulder of the plasticizer oil. The metal of the mould has a catalytic effect on the change in the chemical structure of the oil.