The reaction of substituted phenyl iodosoacetates with toluene under pseudo-first-order conditions is first-order in both components. The marked catalysis by sulphuric acid is explicable by two mechanisms: one depends on the ion pair Phl⁺HSO₄, HSO₄^– and involves base-catalysis, the other on ionisation parallel to the J₀ function. There is a primary hydrogen isotope effect and the reaction is highly substrate-selective between toluene and benzene. The isomer specificity of the reaction is attributed to different stabilisation of the Wheland intermediates for ortho- and para-substitution rather than to steric effects.