Proton conductivities were examined for WO3.2H(2)O (0-40 vol.%)/epoxy resin composite films prepared by the precursor mixing method (i.e., precursor mixing and precipitation Of WO3.2H(2)O after film shaping). The composite films with 16-40 vol% WO3.2H(2)O had large primary and secondary particles Of WO3.2H(2)O compared with WO3.2H(2)O powders obtained by hydrolysis of Na2WO4 solution. The proton conductivity at 50 degreesC under saturated water vapor pressure increased abruptly with WO3.2H(2)O content, reaching the value Of WO3.2H(2)O powder compact at 16 vol.%. The conductivity of the film with 40 vol.% WO3.2H(2)O was 6.3 x 10(-4) S cm(-1), which is higher than that Of WO3.2H(2)O powder compact. The bulk conduction mechanism through the hydrogen bond network Of WO3.2H(2)O Was suggested to be dominant above 40 degreesC. The large primary particles and fully connected secondary particles Of WO3.2H(2)O were assumed to contribute effectively to proton migration in the composite films. (C) 2003 Elsevier B.V. All rights reserved.