This work reports the thermal oxidation of DGEBA resins totally cured with cycloaliphatic (isophorone diamine) or with linear aliphatic (trioxa-tridecanediamine or TTDA) diamine hardener. Chemical changes (carbonyl build-up) were monitored by FTIR and macromolecular changes (chains scission and crosslinking) from Tg variations measured by DSC. Exposition at several temperatures and under several oxygen pressures showed that DGEBA/IPDA has systematically the highest oxidation rate. It was also observed that DGEBA/IPDA undergoes mainly chain scissions whereas DGEBA/TTDA undergoes a predominant crosslinking under air being partially suppressed under enhanced oxygen pressure. Results were discussed on the basis of the possible influence of the structure of hardener.