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Properties of oil and paper in a transformer degrade primarily due to thermal ageing and moisture ingress. Dielectric diagnostic tests, such as Recovery Voltage (RV), and Polarizations and Depolarization Current (PDC) measurement are currently being explored as potential tools for insulation condition assessment. A modern chemical analysis tool for paper molecular weight (MW) measurement, Gel Permeation Chromatography (GPC) or the more accurately described Size Exclusion Chromatography (SEC) promises to be useful in assessing ageing condition. However, the issue of separately assessing the impacts of ageing and moisture on oil and paper has been a key issue for many years. In the current research project, a series of experiments have been performed under controlled laboratory conditions with preset moisture content, and at controlled high temperature ageing. Whereas RV and PDC measurement results were found to be more sensitive to the moisture content of the oil and paper insulation, the MW distribution measurement by SEC provided a trend of insulation thermal ageing. This paper first provides a brief description of RVM, PDC and SEC procedures followed by a description of the experimental techniques adopted. Results are then analysed with the view of separately understanding the impacts of thermal ageing and moisture on the condition of oil and paper insulation in a transformer.