It is suggested that the early stages of oil deterioration progress at a rate which has the characteristics of autocatalysis. This is followed by a prolonged period during which the rate is controlled by the partial pressure of oxygen in the atmosphere. It is shown that the period of autocatalytic acceleration can be shortened by slightly modifying the design of transformers, thus increasing the useful life of the oil. Tests are described which indicate the amount of improvement likely to be gained.Attention is also directed to the behaviour of oil vapour under electrical stress, and to the large quantities of acid and sludge sometimes produced as a result of stress from live conductors, terminals and insulators projecting above the oil surface.