The rate constants for the cleavage of the series of substituted phenyltrimethylsilanes by KOH in aqueous DMSO have been measured, and the pattern of substituent effects is shown to be consistent with a mechanism involving rate determining separation of a phenyl carbanion accompanied by electrophilic assistance from the solvent. A number of heterocyclic trimethylsilyl derivatives have also been cleaved and the rates are discussed in terms of the proposed mechanism. Cleavage also occurs using other highly basic nucleophiles, and there is excellent correlation between the rate constants for cleavage and the acidity functions of the media used. Although all dipolar aprotic solvents greatly enhance the nucleophilicity of small anions because of the absence of solvation, DMSO has the apparently unique ability to facilitate cleavage in the reaction which was studied.