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Deacylation of unsubstituted and of o-F-, m-F, p-F, α-F, pentafluoro-, p-methyl-, and p-trifluoromethyl-substituted trans-cinnamoyl- α-chymotrypsins has been studied from pH 4 to 8. The deacylation rate constants were found to depend upon the ionization state of a group on the enzyme with an apparent pK in the range 6.3–7.3. The hydrolysis rates of the correspondingly substituted p-nitrophenylcinnamate esters were determined at pH 10.6. Correlation of the data for these model reactions with the corresponding rates of enzyme deacylation suggests that the p-methyl-substituted acylenzyme is about an order of magnitude more reactive than expected while p-trifluoromethyl substitution results in deacylation at a rate 10 times slower than expected. The remaining substituents exert about the anticipated rate effect on deacylation.