The paper presents the results of laboratory tests regarding the impact of accelerated thermal ageing of cellulose–aramid insulation samples saturated with electrical-insulating synthetic ester on the polarization and depolarization current characteristics being analyzed in the time domain. In the tests, electro-technical paper from DuPontTM was used, commercially designated as NOMEX®910. Laboratory-accelerated ageing consisted of heating with air a supply of samples of not-impregnated cellulose–aramid insulation for a specified time at specified temperatures. The temperatures and the warming time were selected so as to faithfully represent the degree of the thermal degradation of the cellulose fibers that takes place during normal operation of the material in power transformers. To determine the described impact of the ageing process in the insulation samples on the dielectric properties, the Polarization and Depolarization Method was applied. In the measurements, the MIC-15k1 high resistance meter from SONEL S.A. was used.