Chitosan/polyaniline (PANi) semi-interpenetrating network (semi-IPN) polymers were prepared under the condition of HCl aqueous solution (pH < 1). The structure and half oxidation state of emeraldine base PANi, adjusted to pH < 1, underwent changes of structure and electrical properties. Changes of structural properties were investigated using FTIR spectroscopy, solid-state C-13-NMR, and wide-angle X-ray diffraction. Spectroscopic studies revealed that the PANi was converted to its pH-switched emeraldine salt form. The surface conductivity of the semi-IPN depended on the chitosan/PANi weight ratio and the pH condition. The conductivity of the semi-IPNs increased with increasing PANi content with adjusted pH < 1. (c) 2005 Wiley Periodicals, Inc.