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Frequency domain dielectric spectroscopy, measurements of capacitance and dissipation factor/power factor in a wide frequency range becomes increasingly popular in assessment of power transformer insulation systems. The interpretation of the measurement results is mostly made using a simple model taking the relative amount of oil and impregnated cellulose paper into account. This paper presents main findings from frequency domain dielectric spectroscopy measurements on transformer oil samples from different power transformers still in service. The interpretation model using oil permittivity of 2.2 combined with its conductivity is compared with actual measurements in the frequency range of 1000 Hz down to 0.01, sometimes down to 0.001 Hz. The temperature dependence and possible voltage dependence, especially at lower frequencies, are also investigated.