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Samples of epoxy filled with silica have been subjected to hygrothermal aging at 80°C and 80% relative humidity for several months. These samples were characterized by the measurements of resistances, partial discharges and breakdown voltages. Results for both silanized and non-silanized fillers were obtained. After ageing, large differences were observed in both breakdown voltage and resistance. The breakdown voltage was reduced by a factor of 10 after 74 days of ageing without silanization whereas the decrease was a factor of 2 with silanized fillers. The resistance was reduced by more than 4 decades in the former case and one decade in the second case. The breakdown was preceded by the occurrence of partial discharges. Partial Discharges Inception Voltage (PDIV) dropped by a factor of 10 between 32 days and 74 days of ageing. The PDIV was sensitive to the sample temperature: it was lower at larger temperatures. Partial Discharges (PD) patterns suggest that the discharges occurred in gaseous cavities. Based upon these measurements, a model of breakdown phenomenon based upon cavities filled of water vapour is proposed and discussed.