Polyaniline (PANI) in its emeraldine base (insulating) and salt (conducting) form was investigated by thermogravimetry (TGA) in inert and oxidative atmosphere. Typical two and three step decomposition profiles were obtained. On the basis of isoconversional analysis by the method of Friedman and Ozawa-Flynn-Wall it was found that for all the PANI samples the value of the activation energy (E) is not constant during the degradation and therefore cannot be applied for the description of the overall decomposition process. Further kinetic studies revealed that the model function, describing the mechanism of decomposition, is either of a n-th order reaction, or of Jander's diffusion type. This leads to the conclusion that the degradation mechanism strongly depends on the experimental conditions and structure of the PANI form. The obtained kinetic parameters in the form of E, the pre-exponential factor A and the model function f(alpha) allow a prediction of the polymer's thermal behaviour in an extrapolated range of the degree of conversion (alpha), time (t) and temperature (T).