The embrittlement of metallic systems by hydrogen is a widespread phenomenon but the precise role of hydrogen in this process is not well understood and predictive mechanisms are not available. Here, a new model is proposed wherein hydrogen accumulation around a microcrack tip prevents crack-tip dislocation emission or absorption, and thus suppresses crack-tip blunting and ductile fracture while promoting cleavage fracture. The conceptual model is demonstrated via atomistic simulations of the evolution of equilibrium hydrogen distributions around a crack tip in Ni under increasing applied load, followed by measurement of dislocation emission and/or cleavage. These analyses are performed in single crystal Ni and for several tilt grain boundaries and several initial crack notch radii, using molecular statics and embedded-atom-method interatomic potentials. A kinetic analysis is used to calculate the size of the "nanohydride" formed at the crack tip as a function of hydrogen chemical potential, temperature, H diffusion rate, load level, and loading rate. Combining the kinetic analysis with the deformation/fracture analysis generates a mechanism map that predicts a ductile-to-brittle transition as a function of material and loading parameters. The mechanism map is applied to predict H embrittlement of unnotched tensile specimens of Ni, and the predictions and experiments match well for material parameter values expected to be pertinent in these materials. The mechanism proposed and validated here directly identifies the role of H in driving a change in fracture mode and toughness as a function of material and loading parameters. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.