Stress-relaxation studies show that network scission in peroxide cured methylvinyl silicone rubbers at temperatures up to 250°C is largely the result of hydrolytic reactions in the main chain polymer. There are indications that at 250°C and above oxidative scission of crosslinks makes a significant contribution to the observed rate of continuous stress-relaxation. There is no indication that acidic byproducts of the vulcanization reaction catalyse the hydrolysis of siloxane bonds in the polymer.