The mechanism of the generation of protonic acid sites from molecular hydrogen on WO3-ZrO2 was studied by IR spectroscopy. The spectral changes in the W=O stretching and the Off stretching regions were observed in response to the exposure to hydrogen and the removal of hydrogen. By heating in the presence of hydrogen, the W=O stretching bands at 1021 and 1014 cm(-1) eroded and a broad band around 980 cm(-1) was developed. The peaks at 1021, 1014 and 980 cm(-1) restored to their original intensities on heating in a vacuum. A broad OH stretching band around 3600 cm(-1) was intensified by heating in the presence of hydrogen and restored to its original intensity by removal of the hydrogen from a gas phase. Pyridine adsorption caused essentially the same changes in the W=O stretching region as hydrogen adsorption. Based on the spectral changes, the changes in the structure of acid sites caused by hydrogen adsorption are proposed. (C) 2003 Elsevier Science B.V. All rights reserved.