The present work focuses on the chemical reaction of water with quartz powders undergoing mechanical activation. The reaction produces gaseous hydrogen and is accompanied by a decrease of the water pH. The observed behavior is related to the chemical reactivity of the active sites generated at the surface of quartz powders by fracture and attrition. Their apparent surface density was estimated by connecting the reaction kinetics with the specific surface area of quartz powders. The apparent surface density of the active sites generated by fracture is higher than the one of the active sites generated by attrition. Unexpectedly, it is also higher than the maximum possible surface density of dangling bonds, which suggests for the active sites generated by fracture an unusually high chemical reactivity. This can be regarded as a consequence of undisclosed mechanochemical effects. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.