The hydrolysis of a series of ferrocenyl arylmethyl acetates, FcCH(OAc)(p-XC 6H 4) (Fc=ferrocenyl group; X= OCH 3, CH 3, H, Cl, Br, NO 2), has been studied in acetone-water 4:1 v/v. The logarithms of the first-order rate constant fairly correlate with σ +substituent constants and with E 1 2 chronopotentiometric potentials. However, the sensitivity of the system to the substituent effects on the aryl group, as revealed by a ϱ value of −1.40, is sharply-lower than that of trityl and benzhydryl systems, as well as of heteroannularly substituted ferrocenyl derivatives. The results are interpreted as due to the high electron-releasing effect of the Fc group and to the localization of a considerable positive charge onto the iron atom in the reactions involving the formation of α-ferrocenyl carbocations.