The temperature dependence of the hydroxyl density of silicon oxide is assessed by estimating the Gibbs free energy of hydroxylation of alpha-quartz (111) in equilibrium with water vapour. Surface Gibbs free energies of pristine bulk-truncated terminations, relaxed using spin-density-functional theory, favor oxygen excess at the surface consistent with findings for reconstructed alpha-quartz faces. Hydroxyl densities of 5.6 and 7.4 OH/nm(2) were calculated to depend on temperature in good agreement with experiments. Hence, considering chemical potentials for the gas phase proves to be crucial to interpret the observed variation in reactivity with temperature for silicon oxide. (C) 2011 Elsevier B. V. All rights reserved.