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The insulation system of power transformers consists of oil and cellulose, whose dielectric properties are strongly influenced by moisture, temperature and aging. Their condition can therefore, be evaluated using dielectric response measurements. This contribution describes the return voltage measurement (RVM) diagnostic method based on scanning the polarization spectra of the oil-paper insulation in the time range of 1 s to 1200 s at an excitation voltage of 500 V (DC). The variation of the dissipation factor (tan δ) and power factor (cos θ) in the frequency range of 0.0001 to 1000 Hz were measured and the effects of moisture and aging were analyzed. Furthermore accelerated aging processes of oil-paper samples were investigated under air environment up to a temperature of 388 K (115°C). Typical transformer proportions of copper, aluminum, zinc and iron were added to oil-paper insulation during the aging process in order to simulate the possible oxidation processes. For the investigation of the long time behavior, the oilpaper insulation samples were aged at different aging times at 250, 500, 1000 and 2000 hours which simulate long time service condition due to the used severe aging conditions.