This paper describes a d.c. resistance test which has been developed in connection with the maintenance of transformer and switch oils on the Central Electricity Board's system. Details of the testing technique are given, and the results of resistance measurements made on representative samples of oil in various stages of deterioration are discussed.It is shown that the d.c. resistance is related to the power factor sufficiently closely to enable either parameter to be used equally well as an indication of the characteristics of the oil. Data are given on the variation of resistance with temperature, and it is found that, for used oils of relatively low resistance, the logarithm of the resistance is proportional to the reciprocal of the absolute temperature.It is demonstrated that d.c. resistance measurements can be used as a criterion of oil condition if the following limits of resistivity at 20°C are adopted:—“Good”—Greater than 20 × 106megohm-cm.“Fair”—From 2 to 20 × 106megohm-cm.“Bad”—Less than 2 × 106megohm-cm.It is concluded that the resistance test gives more information about the condition of used oil than any other single test, and it is suggested that consideration should be given to its adoption as a standard test.