The two title reactions were investigated by means of ab initio molecular orbital calculations. Under neutral conditions, the hydrolysis of methylmethoxydihydroxysilane was examined, and the water-trimer cluster was found to be a reactant for ready bond interchange. The condensation of methyltrihydroxysilane was caused by the participation of the water dimer. Two of the observed reactions involved a front-side nucleophilic attack to the silicon center or a similar mechanism. Acid-and base-promoted hydrolyses were also examined, and their enhanced reactivities were ascribed to the stability of the silyl cation and a hypervalent intermediate, respectively.