We have performed a theoretical study of the decomposition of the five-coordinate silicon anions (siliconates) H3(CH3)SiOH- and (CH3) 4SiOH-. These ions can be formed from hydroxide ion attack on methylsilane or tetramethylsilane, respectively. Both MNDO and ab initio calculations show that removal of methane from the five-coordinate structure is likely, via a transfer of the proton from the OH group to an adjacent methyl substituent. This process we term anionic dissociative proton transfer, because the transition state is one in which the methyl group being removed is almost completely dissociated to methide ion before the proton transfer takes place. This conclusion is in agreement with earlier experimental work by DePuy, Bierbaum, and Damrauer, which indicated a correlation between the rates for these reactions and the gas-phase acidities of the products. Detailed comparisons among the MNDO calculations, the ab initio calculations, and available experimental data indicate that great caution should be exercised in using MNDO calculations to draw even semiquantitative conclusions about these silicon-containing anion systems without resorting to reliable ab initio results on similar systems.