PVA/WO3 hybrids were produced by using ion exchange process of Na2WO4.2H(2)O. Microstructures of this hybrid was estimated by SAXS measurement as follows; the WO3 rich domains having radius of gyration of about 5.7 nm (estimated in Guinier regime), in which PVA chains were partly incorporated, were dispersed evenly in PVA matrix with the correlation length of 20-30 nm (approximated by Bragg equation). The electron density difference between WO3 and matrix, Deltarho, was rather high and sharp, and this indicates that the domains dispersed in PVA matrix is rich in WO3 component. The physical properties of the hybrids were affected by the morphology. The mechanical properties of the hybrid were improved markedly, e.g. the tensile strength increased from 35 MPa (PVA) to 55 MPa at 27 wt% Of WO3 content, and the modulus also increased above T, with increasing amount Of WO3. A tan delta peak appeared at 75 degreesC shifted to the higher temperature side with WO3 content. These were caused by strong interaction between PVA chains and WO3 domains, which was formed by incorporation of PVA chain into WO3 domains having hydrogen bonding. This hybrid was transparent and showed the photochromism especially at low concentration Of WO3. When UV-visible light of wavelength above 350 run was irradiated on the hybrid, a minimum occurred at 630 nm in the UV-visible spectrum, and color changed from yellow to blue. Bleaching occurred in the dark in 100% relative humidity. Consequently, the ion exchange process was suitable to obtain PVA/WO3 hybrid, since this procedure was simple and economical, and hybrid thus produced had excellent mechanical and photochromism properties. (C) 2003 Elsevier Science Ltd. All rights reserved.