Distinct thermal transitions have been observed in nanocomposites based on iron oxide-polypyrrole prepared by simultaneous gelation and polymerization process. The transition behaviour for various iron oxide-polypyrrole compositions was investigated with the help of differential scanning calorimetery. It has been observed that the samples show a magnetic to nonmagnetic transition at 410 degreesC. The presence of exothermic peak at 410 degreesC is a strong indication of magnetic transition corresponding to gamma-Fe2O3 phase of iron oxide to a nonmagnetic alpha-Fe2O3 phase in these nanocomposites. However such a transition was not observed in nanocomposites containing lesser concentration of conducting polymer. (C) 2002 Elsevier Science B.V. All rights reserved.