The ac surface ageing of high temperature vulcanized (HTV) silicone rubber, ethylene propylene rubber (EPR) and cycloaliphatic epoxy is investigated in a salt-fog chamber. The silicone rubber (SR) and epoxy samples are filled with alumina trihydrate (ΑΤΗ). The EPR samples are filled with either ΑΤΗ or silica. The filler concentration is varied over a wide range. Testing is conducted at three electrical stresses centered around the average value used in practice and a microprocessor based system is used for data acquisition.