The heat capacity of activation for the solvolysis of t-butyl chloride in the high water range of alcohol-water mixtures shows dramatic changes in value over the alcohol concentration range where quasi-aqueous structure is replaced by other solvent interactions. The corresponding changes in ΔH‡ and ΔS‡ were found to be less complex than previously reported. The mechanistic implications of the changes in quasi-thermodynamic terms are discussed. A brief description is given for an automatic, random-access system for rate determination by conductance. © 1969, American Chemical Society. All rights reserved.