The hygrothermal conditioning of an epoxy resin at 80degC under 80% RH has been followed by weight measurements, therm ogravimetric analysis (TGA) and dynamical mechanical analysis (DMA). The samples are either filled with 60% by weight of silica flour or are not filled. Above an apparent saturation value of about 1.5% reached within a few days, a slight but significant mass uptake was observed in the filled resin, especially after 50 days. The TGA showed an evolution of the filled samples with conditioning after 50 days as well, which was not observed on unfilled samples. For the filled samples, the elastic modulus in the rubbery state decreased with conditioning. These evolutions have been attributed to the formation of a degraded inter-phase region due to hydrolysis occurring after the debonding of the filler-matrix interface caused by the absorbed water.