Substituent effects on the proton dissociation of protonated aniline derivatives upon femtosecond laser pulse excitation are investigated for m- and p-cyanoanilines and m- and p-methoxyanilines in aqueous solution. The cyano substitution at the meta-position increases the rate significantly (k(dis) = 3.7 x 10(11) s(-1)) compared to aniline (k(dis) = 1.3 x 10(10) s(-1)), while the methoxy substitution reduces the rate remarkably. Either substituent at the para-position shows only slight influences on the rate. The strong dependences of the k(dis) value on the substituent and the substituted position are reasonably explained on the basis of the free energy change. (C) 2003 Elsevier B.V. All rights reserved.