In the course of the last ten years the author has investigated a considerable number of accidents and dangerous occurrences due to the explosive failure of oil circuit-breakers and oil-immersed control gear, and in doing so it has become apparent that not all were due to inadequate rupturing capacity or defective insulation. This has been particularly true of a number of failures in recent years of the control gear for arc furnaces. Some of these have been due to defective mechanical design and steps now being taken will, it is hoped, go a long way to cure this. The present paper, however, is an attempt to assess what part deterioration of the switch oil may play. The importance of the problem may be judged from the fact that oil in such switches is now frequently changed every 2 or 4 weeks and at some works once a week.Many engineers responsible for the maintenance of switchgear have felt that while the tests laid down in British Standard Specifications may eliminate many new oils which are definitely unsuitable they provide no certain guide to the rejection of used oil after a period of service. A section has therefore been added in which the author suggests the initial tests he himself would take in the search for a more suitable basis if he had the necessary laboratory facilities.The subject is one in which there is little agreement among experts, but the author feels that the parer will have completely fulfilled its purpose if it succeeds in giving a new impetus to research and possibly a new direction, even if the tentative conclusions set out here have later to be discarded.