A simplification of a previously presented “closed-loop” kinetic scheme of radical chain oxidation consisting in taking into account the substrate consumption is applied to the thermal oxidation (in oxygen excess) of a polybismaleimide (BMI) and an amine crosslinked epoxy (ACE). The model fits gravimetric curves in the 0–12% mass loss range well, for both polymers and constitutes a noticeable progress compared to the previously established “steady-state” models. The qualitative difference between BMI (presence of an initial maximum) and ACE (continuous mass decrease), is explained by the fact that ACE hydroperoxides are considerably less stable than BMI ones, which can be mechanistically justified. A generalisation of these results leading to a classification of gravimetric oxidation curves according to their shape, is tentatively proposed.