Standard free energies of transfer from ethyl benzoate to 17 other solvents of trimethylamine, p-nitrobenzyl chloride, and the trimethylamine–p-nitrobenzyl chloride transition state have been calculated (at 298 K). The transition state is stabilised by polar solvents and is destabilised by non-polar solvents; dipolar aprotic solvents are much more effective in stabilising the transition state than are the aliphatic alcohols. Similar calculations have been carried out for the substitution of o-, m- and p-nitrobenzyl chlorides by trimethylamine for 6 solvents (at 303 K). A comparison of solvent effects on the free energy of the trimethylamine–p-nitrobenzyl chloride transition state, the t-butyl chloride solvolysis transition state, and the Et₄N⁺l^– ion pair indicates that the solvolysis transition state markedly resembles an ion pair in behaviour, but that the transition state for substitution of p-nitrobenzyl chloride by trimethylamine is very much less polar.