Rates of solvolysis are presented for a number of polar substrates in light and heavy water from which the rate ratios [Formula: see text] may be obtained. Compounds studied include methyl, o-methyl-, p-methyl-, p-methoxy-, and p-fluoro-benzenesulphonate, the α- and β-methylation series of alkyl methanesulphonates, methyl and isopropyl nitrate, methyl phosphate and sulphate, methane- and benzene-sulphonyl chloride, and t-butyldimethylsulphonium iodide. The oxyanion esters, despite wide variation in reactivities, show very small variation in the rate ratios which lie near 0.9. The values for the chlorides are 0.63, and for the sulphonium salt, 0.95. The failure of the rate ratios to reflect the mechanistic changes required by variation in the alkyl groups and oxyanions is attributed to the structure of initial state solvation and the easy saturation of ratio-affecting structural changes by small charge development in the transition state.