The arbitrary conditions of electrical stress and temperature adopted by the electrical industry when assessing the gassing tendencies of insulating oils have been examined closely. This had led to the important conclusion that oils commonly accepted as ‘gas-absorbing’ can evolve gas if suitable conditions exist. With the possibility of increasing temperatures and stresses in transformer insulation in the future, investigations have been carried out on their influence on the gassing characteristics of oil. It is clearly evident on the basis of this work that, in defining an oil as ‘gas-absorbing’ or ’gas-evolving’, the parameters of the gassing test must be fully defined, not only with regard to the gas phase but also in respect of the temperature stress applied.