The effect of heating of polyaniline (PANI) at 80 degrees C on its electrochemical activity is studied. Although the thermogravimetric analysis indicates that PANI is stable at temperatures up to about 250 degrees C and it undergoes decomposition at higher temperatures, its intrinsic redox electrochemical activity decreases with duration of heating at a temperature as low as 80 degrees C. The polymer completely loses its electrochemical activity. This behavior of PANI is studied by recording cyclic voltammetry and electrochemical impedance spectroscopy at different intervals of heating time. Furthermore, the electrochemical activity of PANI towards the redox reaction Fe2+/Fe3+ also decreases similarly. The decrease in electrochemical activity of PANI is attributed to an irreversible loss of water molecules. The reaction order degradation of PANI is found to be close to unity, and a value of 1.44 x 10(-4) s(-1) is obtained for the rate constant. The deactivated PANI does not recover its electrochemical activity even after a prolonged treatment in acidic electrolytes. (c) 2006 Elsevier B.V. All rights reserved.