The kinetic study of the hydrolysis of p-nitrophenylacetate in the presence of primary thiols indicates the thiolate anion as the sole catalytic species. Comparison of the true second order rate constants (k RS-) reveals that purely aliphatic primary thiols behave differently from aromatic α-substituted primary thiols. In the latter group a correlation can be established between the true second order rate constants and the p K SH values by ...