The effects of water density on the acid-catalytic properties of TiO2 and WO3/TiO2 catalysts in supercritical water at 400 degrees C were investigated by using the kinetic analysis of the dehydration reaction of glycerol. The reaction selectivity of TiO2 and WO3/TiO2 catalysts and the apparent-reaction orders for water indicated that the acid-catalytic properties of these two catalysts show different dependence on water density. In the reaction using TiO2, the contribution of Lewis acid sites in TiO2 was large at a low water density, while the contribution of Bronsted acid sites in TiO2 increased with increasing water density. On the other hand, the reaction using WO3/TiO2 was mainly catalyzed by Bronsted acid sites in WO3/TiO2 even at a low water density, and the nature of Lewis/Bronsted acid sites in WO3/TiO2 was not influenced by the water density. (C) 2013 Elsevier B.V. All rights reserved.