Both the loss tangent and the d.c. conductivity of insulating oils decrease with time after the oil is introduced into the test cell, in the absence of electric stress and at room temperature.When the cell is heated immediately after filling, the loss tangent and d.c. conductivity rise to a maximum, but decrease to a much lower value with time at constant temperature. The maximum value, the rate of rise to that value and the subsequent decrease with time are functions of the thermal capacity and shape of the cell, the technique used to clean the cell, the temperature and the oxygen content of the oil. The subsequent decrease in dielectric loss is thought to be due to catalytic oxidation of some component in the oil at the metal-electrode surface, although it is possible that ions may be merely trapped at this surface.The effects have been observed with a low-viscosity cable impregnant, both unused and severely contaminated, a transformer oil and a synthetic hydrocarbon.The implications of the effect are discussed in relation to routine testing of oils, to investigations of the effects of contamination in oils and to work on oil-impregnated paper. It is possible that the effect may be partially or wholly eliminated by the use of plastic-coated electrodes.