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Moisture is one of the most influencing parameters which accelerates solid insulation degradation process in transformers. Correct estimation of the moisture within solid insulation of transformer is still a challenge due to temperature driven complex moisture dynamics between oil and paper insulation. Although some authors have proposed coefficients for Kraft paper as well as for non-impregnated pressboard much less work has been done on oil-impregnated pressboard insulations. In this paper, the determination of the diffusion coefficient of moisture in transformer pressboard impregnated with mineral oil is tackled. Studies were carried out on different samples thickness of pressboard which are typical in power transformer. The experimental conditions were simulated using a mathematical model based on the Fick¿s second law and an expression for the diffusion coefficient is proposed and validated experimentally.