The acid catalytic properties of TiO2, Nb2O5, and NbOx/TiO2 in supercritical water at 400 degrees C and 25-65 MPa were investigated. Two model reactions were used to elucidate the effects of water properties on catalysis and their variation with the catalysts. The kinetics of both reactions obeyed the Langmuir-Hinshelwood model, and the competitive adsorption of water suppressed these reactions. An increase in the ion product of water with increasing water density promoted these reactions, particularly with TiO2 and Nb2O5. In addition, temperature programmed desorption suggested that active sites with weaker affinity for water are mainly active in supercritical water, wherein Nb2O5 shows Bronsted acidity, NbOx/TiO2 shows Lewis and Bronsted acidity, and TiO2 tends to show Lewis acidity. The Bronsted acidities of TiO2 and NbOx/TiO2 increased with increasing water density. These findings will allow us to predict the acidities of solid acid catalysts in supercritical water.