Polyaniline (PANI):polyethyleneterephthalate (PET) composite was prepared by chemical polymerization of aniline diffused in the PET matrix. Thus prepared composite films were characterized by fourier-transform infrared spectroscopy and scanning electron microscopy and their electrical properties and the thermo-oxidative stability was studied by thermogravimetry and differential thermal analysis. The stability in terms of DC electrical conductivity retention was studied in an oxidative environment by two slightly different techniques viz. isothermal and cyclic techniques. DC electrical conductivity of composite films was found to be stable up to 90 degrees C for most of the composites under ambient conditions. The composite films were employed as cathode material in secondary cells containing 1M ZnCl2 solution. The studies were carried out on the charge/discharge cycles under a constant current load 140 mA. The composite films showed similar behavior in electrolyte solution and cell response is reversible. To determine the diffusion coefficient for the chloride ions diffusion into the composite films electrochemically, galvanostatic pulse method was used. The diffusion coefficient was estimated to be similar to 3.28 x 10(-12) cm(2) s(-1). (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1366-1375, 2010