The stator windings of large air-cooled rotating machines are mainly insulated with mica-glass fabric impregnated with epoxy resin. In service, these materials are submitted to thermal, mechanical and electrical stresses. A specific aging arrangement has been set-up allowing artificial thermo-oxidative aging of samples, thanks to the application of a dry O2 pressure at a controlled temperature. Three different materials, one reference material and two candidate materials, of different thermal classes or different thermal conductivity, have been aged and compared. Partial Discharges (PD) analysis of the samples has been conducted and correlated with samples' morphology. Because of the small size of the samples, a specific PD measurement arrangement has also been set-up. After only a few hundred hours aging, delaminations of a few tens of μm were observed within the samples. PD measurements revealed a decrease of the PD Inception Voltage (PDIV) on these aged samples. The PDIV on samples with specific cavity thickness were also in good agreement with Paschen law. PD measurements, especially the PDIV, seem to be a good indicator of the progress of the aging.