Five different polyaniline salts have been prepared by chemical polymerization of aniline in aqueous solution of different acids. Polyaniline samples have been heat treated at four different temperatures (150, 200, 275, and 375 degrees C) and characterized by electron paramagnetic, electronic absorption, and infrared spectral measurements. Thermal stabilities of the chemically synthesized polyaniline salts have been studied by thermal analysis and spectral methods. Polyaniline salts undergo a three-step weight-loss process in the heating cycle. The first step (up to 110 degrees C) corresponds to the loss of water molecules from the polymer chain. In the second step (110-275 degrees C), a small amount of acid escapes as volatile gas, and after 275 degrees C the polymer undergoes oxidative thermal degradation in the third step. It was found that thermal stability of polyaniline salts depends on the counteranion used and the polymer is apparently stable up to 250 degrees C. No structural changes have taken place up to 200 degrees C and this has been confirmed from infrared and electronic absorption spectra. No definite correlation exists between conductivity and spin concentration.