HCl-doped polyaniline (HCl-PANI) powder was mechanically mixed with Fe nanoparticles to obtain the HCl-PANI-Fe composite. The composite pellets containing 30 and 50 wt% Fe nanoparticles were heated in air at 50, 100, 150 and 200 degrees C for times up to 60 min. The heat treatment degrades the crystallinity of the HCl-PANI and promotes the oxidation of the Fe nanoparticles. After heat-treating, the aromatic structure of PANI is retained in the HCl-PANI and HCl-PANI-Fe pellets. However, the heat treatment causes the dopant dedoping from the polymer backbones. The HCl-PANI-Fe composite pellets heated are in a higher reduction state compared with the unheated ones. Thermogravimetric analysis reveals that a weight loss decreases with increasing the Fe nanoparticle content at the same heating temperature. The weight loss increases rapidly within the first 5-min isothermal time at 200 degrees C. The conductivity decreases with increasing the heat-treating temperature. A rapid decrease in the conductivity happens within the first 20-min heat-treating time for the HCl-PANI-30 wt% Fe composite and within the first 10-min heat-treating time for the HCl-PANI-50 wt% Fe composite at 200 degrees C. The composites exhibit a narrow hysteresis curve typical for superparamagnetic nanoparticles and the magnetization behavior is independent of the heat-treating conditions. (C) 2009 Elsevier B.V. All rights reserved.