The water formation rate on platinum and palladium as a function of the reactant pressures at the catalytic surface has been determined from measurements of the three-dimensional hydrogen pressure distributions over the catalytic surface. Measurements were made for gas mixtures containing less than 1% of hydrogen and 20% of oxygen in argon at atmospheric pressure. The experiments were performed in a reactor with a simple and well-defined geometry which makes it possible to calculate the hydrogen pressure distribution in the entire region where measurements are made. The water formation rate as a function of the hydrogen pressure at the catalytic surface is obtained by comparing calculated hydrogen pressure distributions to measured ones. It is concluded that three-dimensional reactant pressure distributions can be used to accurately determine the reaction rate on a catalytic surface as a function of the reactant pressures at the surface. This is valuable for studies in the viscous pressure regime, where mass transfer limitations may be difficult to avoid. © 2001 Elsevier Science B.V.