The mechanism for the reaction of H2O- with C2H4 has been investigated theoretically at the UCCSD(T)/6-31++G(d,p)//UB3LYP/6-31++G(d,p) level. Two major products (A) H2O + C2H4- and (B) HO + C2H5+ are shown to be formed through the parallel reactions of charge-transfer and proton-transfer via the respective intermediates of (a) H2O (.) C2H4-and (b) HO (.) HC2H4-. The dissociation of intermediate (c) HOC2H5+ resulting from the 1,3-H-shift of (a), may lead to the minor product (C) HOCH2- + CH3. The calculated results agree well with the available experimental data and may be helpful for understanding the chemical behavior of analogous radical cations containing H-O bond. (C) 2002 Elsevier Science B.V. All rights reserved.