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This paper examines the influence of moisture and temperature on the degree of polymerization (DP) and furanic compounds. At normal service temperatures, the average moisture content in the solid insulation of transformers has a much stronger influence on the development of furanic compounds and degradation of the insulation than temperature itself. The influence of the initial moisture in the solid insulation on aging under normal transformer service temperatures of 75, 85, and 95 °C has been investigated experimentally. Wet transformers have usually higher levels of furans, inferior oil properties, and lower DP values of paper than dry ones under similar service conditions. The development of furan concentration and moisture content are very important aging indicators for the evaluation and life assessment of transformer in service. Further work is necessary to combine these important aging characteristics into a valuable tool for the assessment of a large aging transformer population.