Polyaniline/cobalt oxide composites were synthesized by an in situ chemical polymerization method with ammonium persulfate as an oxidizing agent. This was a single-step polymerization process for the direct synthesis of the emeraldine salt phase of the polymer. The polymers were characterized with X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectral analysis. The formation of mixed phases of the polymer together with the conducting emeraldine salt phase was confirmed by spectroscopic techniques. High-temperature conductivity measurements showed thermally activated behavior. A change in the resistance was observed with respect to the relative humidity when the pellets were exposed to a wide humidity range of 10-95%. (c) 2006 Wiley Periodicals, Inc.