Hydrolysis of Si−O−Si linkages of β-cristobalite by a single H2O molecule is studied within the cluster approach at the DFT (B3LYP) and MP2 levels of theory. The 6-31G(d) and 6-311G(d) basis sets are used. Cluster models, including from 6 up to 14 Si atoms, of the (001) and (111) surface planes are considered. These models are specially designed to take into account the steric constraints imposed by the solid matrix on the Si−O−Si linkages and their nearest surroundings. For comparison, the hydrolysis of the Si−O−Si bridge of the free (HO)3Si−O−Si(OH)3 molecule is also calculated. The computed activation energy of the reaction (ΔEa) for the (001) and (111) planes of β-cristobalite is larger by 5 and 16 kcal/mol, respectively, than for (HO)3Si−O−Si(OH)3 (17 kcal/mol). The higher energy barrier for the surface is due to the resistance of the lattice to the relaxation of the activated complex of the reaction. The difference in ΔEa between the (001) and (111) planes suggests that the larger the number of Si−O...