Solvolysis of t-butyl and t-amyl iodides in aqueous ethanol and aqueous hexafluoropropan-2-ol is catalysed by iodine. The same effect, but to a smaller extent, is also found for t-butyl bromide in aqueous ethanol; the strongest catalysis by iodine was found for the aqueous ethanolysis of 1-adamantyl iodide. The catalytic constants for aqueous ethanolysis of t-butyl iodide decrease less steeply than the rate constants of the uncatalysed reactions as the proportion of ethanol in the medium is increased. The results are accommodated by a mechanism involving pre-association of iodine with the alkyl halide superimposed upon a general mechanism for solvolysis the rate-determining step of which may be ionization, ion-pair separation, or reaction of reversibly formed intimate ion-pairs with the solvent according to the nature of the alkyl halide and the solvolytic medium.