The oxidation of chemically synthesized polypyrrole powders, doped with arylsulfonates, was investigated by thermal analysis. At high temperatures (typically T > 230 degrees C) oxidation appears to be a bulk phenomenon, which leads to the decomposition and complete combustion of the polymer. At moderate temperatures (typically T < 230 degrees C) oxidation is a slow gradual process, most likely limited by O-2 diffusion in the oxidized material. The kinetics of this oxidation process was determined and its activation energy was evaluated to be about 110 kJ mol(-1). At its early stage, this oxidation was shown to induce a proportional, but enhanced, decrease in the conductivity of the material.