Composite materials containing amorphous Cu nanoparticles and nanocrystalline Cu2O embedded in polyaniline matrices have been prepared by a sonochemical method. These composite materials were obtained from the sonication of copper(II) acetate when aniline or 1% v/v aniline–water was used as solvent. Mechanisms for the formation of these products are proposed and discussed. The physical and thermal properties of the as-prepared composite materials are presented. A band gap of 2.61 eV is estimated from optical measurements for the as-prepared Cu2O in polyaniline.