Increasing demands on electriciiy supply systems are focusing attention on the future requirements of insulating materials. Their performance, when subjected to operating temperatures and stresses higher than those currently in use, is of interest,1and development work has been carried out to improve the correlation between laboratory tests and service performance.This paper describes investigations into the life,of insulating oils in transformers operating at top-oil temperatures of 100–105°C. The results obtained indicate that an inhibited oil will give approximately twice the life of an uninhibited oil when operating under severe temperature conditions.