The thermal oxidation of poly(bismaleimide) of the F655-2 type, supplied by Hexcel-Genin, was studied by isothermal gravimetry at 180, 210, and 240 degreesC and various oxygen pressures ranging from 0 to 1.2 bar. Comparison of various sample thicknesses and visible microscopy observations on bulk aged samples shows that the whole oxidized layer has a depth of about 75 mum at 240 degreesC, 138 mum at 210 degreesC, and 229 mum at 180 degreesC. An attempt was made to build a kinetic model to predict this depth. It is based on a differential equation in which O-2 diffusion and its consumption rate, r(C), are coupled, C being the O-2 concentration. Its resolution needs two sets of experiments: the first one to determine the O-2 diffusivity and solubility in the polymer, and the second one to determine r(c). The mathematical form of r(C) is derived from a mechanistic scheme of radical chain oxidation in which initiation is mainly due to POOH decomposition. This expression contains two kinetic parameters, alpha and beta, the values of which are determined from the experimental curves of mass loss rate against O-2 pressure (in the stationary state). The theoretical predictions, at each temperature under consideration, are in excellent agreement with experimental results.