Abstract: Curing of plastic material or rubber for use as electrical insulation is effected by mixing with the plastic or rubber composition 0.1 to 25 weight percent of a crosslinking agent and subjecting the resulting mixture to a UHF electric field. The crosslinking agent used has the characteristic of being excited by the UHF electric field to produce radicals which effect crosslinking of the plastic or rubber composition and of decomposing or combining with the plastic or rubber in the crosslinking process to form decomposition or reaction products which are not excited by an alternating current electric field. Hence, the crosslinking agent does not adversely affect the electrical insulating characteristics of the crosslinked material even when used for insulating alternating current circuits. The composition is free of conventional fillers such as carbon black which retain their electrical activity.