The molecular structures of disiloxane, silanol and water have been determined by a local density functional method employing Gaussian bases. Bond lengths and angles are found to be within a few per cent of the observed values. A 4% expansion of the Si–O bonds in disiloxane is sufficient on energy grounds to allow hydrolysis to occur. These results are used to support the Griggs-Blacic-Frank models of hydrolytic weakening in quartz and to justify the use of an interatomic potential used in a previous investigation.