The influence of a Sigma 10(11 (2) over bar4)/[1 (1) over bar 00] grain boundary in magnesium on the dissociation of the hydrogen molecule has been investigated by ab initio calculations. It is found that the dissociation barrier is indeed lowered by 0.115 eV, including zero point motion, compared to the clean surface. This results in a 25 fold increase in the transition rate, at desorption temperature of 572 K. Furthermore, different diffusion paths of hydrogen in and out of the grain boundary plane are investigated. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.