The effects on the rates of cleavage of 3-ClC6H4CH2MMe3 and 3,5-Cl2C6H3CH2MMe3 compounds with M = Si and Sn have been examined for (a) various concentrations of NaOMe-MeOH; (b) various concentrations of H2O in NaOHH2O-eOH; and (c) variation of R from Me to Et, i-Pr and t-Bu for metal alkoxide-ROH mixtures. Rates have also been measured for cleavages in NaOCH2CF3HOCH2CF3-Me2SO. In all cases there are differences in behaviour between the silicon and tin compounds which are consistent with the postulated difference in the mechanisms of their reactions.