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−Si bridges for a Si atom (2 for the (001) plane and 3 for the (111) plane), the stronger the resistance of the solid matrix to the hydrolysis of a Si−O−Si bridge. This finding allows for the atomic-level substantiation of the earlier hypotheses that (i) the hydrolysis of the first Si−O−Si linkage of a Si atom should be the rate-limiting step for the release of Si(OH)4 and (ii) the dissolution should preferentially take place for the low-linked Si species of the surface. The OH groups produced by the reaction form H-bonds with the nearby Si−OH and Si−O−Si surface species. For both planes, the energy of the reaction (ΔEr) is within the 1−2 kcal/mol range.