The hydrolysis rate of Zn particles by up to 50 mol% water vapor in Ar gas was measured by thermogravimetric analysis at atmospheric pressure and 330-360 degrees C and quantified by a core-shell model. An initial ZnO layer led to an initially linear conversion profile attributed to a fast surface reaction (half-order with respect to water vapor mole fraction, y) followed by a parabolic conversion profile independent of y but dependent on Zn ion diffusion through a ZnO layer. The latter is most important for solar H-2 formation by the Zn/ZnO water-splitting cycle as it determines the required process residence time for Zn hydrolysis. A ready-to-use equation for calculation of ZnO and H-2 formation during Zn hydrolysis is proposed and compared to literature data revealing enhanced hydrolysis rates for submicron Zn particles. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.