The product branching ratios from the infrared multiple-photon dissociation (IRMPD) of a series of tertiary alkoxides are examined and an overall order for the relative acidities of several alkyl groups is inferred. Comparison with the order of acidities derived from a similar silane system reveals an overall similarity, although small differences exist. A model system, the reaction of phenyltrimethylsilane plus hydroxide to give either benzene or methane, is studied more closely. Analysis of the energetics of the alkoxide system and the silane system using statistical reaction rate theory (RRK and RRKM) reveals differences between the mechanisms of decomposition for the systems. These discrepancies between the known, experimental product ratio and the energetics suggest that the transition state of pentacoordinate hydroxysilane anion decomposition has concerted, albeit polar character. Decomposition of the pentacoordinate adduct formed by the reaction of methoxide plus phenyltrimethylsilane is consistent with this suggestion. Caution should be used when inferring the relative acidities of alkyl groups from kinetic methods when assumptions of the decomposition mechanism have not been validated.