Second-order rate coefficients and activation parameters are reported for the substitution of tetramethyl-, tetra-n-propyl-, tetra-n-butyl-, and tetraisobutyl-tin by mercuric chloride in methanol–water mixtures of mole fraction methanol 1, 0·91, and 0·72. In addition, an upper limit can be set for the rate coefficient for substitution of tetraisopropyltin by mercuric chloride. The rate coefficients decrease in value markedly along the series R₄Sn = Me₄Sn Et₄Sn > Prⁿ₄Sn Buⁿ₄Sn > Buⁱ₄Sn Prⁱ₄Sn for substitutions in the three solvent mixtures used. Although the rate coefficients for substitution of any given tetra-alkyltin increase considerably with increasing water content of the solvent, relative rate coefficients compared with that for the substitution of tetramethyltin are but little affected by the change in solvent composition. It is concluded that substitution of the tetra-alkyltins by mercuric chloride, in the three solvent mixtures used, proceeds by the S_E2(open) mechanism of electrophilic substitution at saturated carbon through transition states in which considerable charge separation has occurred.