Kinetic data for solvolyses of α-methylthioacetyl chloride, phenylthioacetyl chloride and thiophene-2-acetyl chloride in at least 33 aqueous solvent systems including 2,2,2-trifluoroethanolâ ethanol solvent were determined at 10 °C by a conductimetric method, and their rates of solvolyses were correlated using Grunwaldâ Winstein type equations with the ionizing power parameter (YCl: based on the solvolyses of 1-adamantyl chloride) and the nucleophilicity parameter (NT). Kinetic solvent isotope effects (KSIEs) in water and methanol were investigated for the above compounds and in methanol also for phenyl, diphenyl, and trimethylacetyl and isobutyryl chlorides. The results show similar absolute rate constants, similar large amounts of nucleophilic solvent assistance, but different KSIE values (explained by general base catalysis).