A series of polyaniline films have been coated on electrodes by chemical and electrochemical means in order to investigate the thermal stability of the electroactivity in air and nitrogen atmospheres. Polyaniline was also synthesized chemically and cast as a membrane (thick film). It was found that the electroactivity and conductivity of the films was stable up to 150 degrees C. Exposure to higher temperatures leads to a decrease in electroactivity and conductivity, although use of a nitrogen atmosphere had less effect on the properties. Thermogravimetric analysis profiles confirmed that the polymer was stable to above 350 degrees C. The changes in the electroactivity may be attributed to morphological (annealing) effects in the polymer and to loss of moisture and dopant ions.