Abstract: Method of assessing the remaining lifetime of a transformer operating at temperatures above IEC 60° C./65° C./78° C. or IEEE 65° C./65° C./80° C. including a core, a winding conductor covered by high temperature insulation material and pressboard material arranged as an insulation barrier and/or a supporting structure. The covered winding conductor and the pressboard material are immersed in a fluid having a fire point of at least 300° C. The method includes at least twice measuring the temperature of the fluid at its top surface and registering the time of each measurement and assessing the remaining lifetime of the transformer as a function of the measured temperatures and the corresponding registered times, and/or obtaining a sample of pressboard material that has been in contact with the fluid at its top surface and analysing the sample to assess the remaining lifetime of the transformer.