Acidity and resistivity measurements have been used to judge the condition of used oil and have revealed a few oils with conductivities which are abnormally high when compared with their low acidities. Examination of such oils by filtration, separation and chemical extraction, together with attempts to simulate the high conductivity by additions of pure substances to new oil, suggested that low resistivity was associated with high iron content.To determine the extent of the problem a survey was carried out on oil samples from Grid and generator transformers with ages ranging between one and 24 years. A statistical analysis of the resistivities, acidities and iron contents of these oils showed that the combined presence of iron compounds and acids, which is more often encountered generator transformers, can give rise to high conductivity.Synthetic insulating fluids, and particularly the chlorinated diphenyls, may be used where low fire risk or high operating temperatures are required. Brief mention is made of a laboratory investigation of these substances which gives some idea of the problem involved when hydrochloric acid is released by a spark occurring in them.