Properties of acidic sites on WO3-ZrO2 and Pt/WO3-ZrO2 were studied. The IR results of adsorbed pyridine were related to effects caused by contact with hydrogen. For both WO3-ZrO2 and Pt/WO3-ZrO2, the ratio of Lewis acid sites to protonic acid sites increases with an increase of the temperature of pretreatment with hydrogen followed by outgassing. All of the protonic acid sites were strong; pyridine molecules remain on the protonic acid sites after outgassing at 673 K. In addition to strong Lewis acid sites, there existed a number of weak Lewis acid sites from which pyridine molecules were desorbed in the temperature range 423-673 K. When WO3-ZrO2 and Pt/WO3-ZrO2 were heated in hydrogen, protonic acid sites were formed with a concomitant decrease of Lewis acid sites. The protonic acid sites thus formed were eliminated by heating in a vacuum, with restoration of Lewis acid sites. The presence of Pt facilitated the interconversion between Lewis acid sites and protonic acid sites; with Pt/WO3-ZrO2, the formation and elimination of the protonic acid sites occurred even at room temperature. Hydrogen dissociation monitored by H-2-D-2 exchange took place over WO3-ZrO2 above 373 K, and was strongly retarded by pyridine preadsorption. These results indicate that the active sites for hydrogen dissociation on WO3-ZrO2 are acidic sites.