Direct ab initio trajectory calculations have been applied to a typical microsolvated S(N)2 reaction F- (H2O) + CH3Cl at hyperthermal collision energies. The branching ratios for the product channels, F- (H2O) + CH3Cl --> Cl- + H2O + CH3F channel I, --> Cl- (H2O) + CH3F channel II --> Cl- + CH3F-H2O channel III, were calculated as a function of center-of-mass collision energy (E-coll). It was found that branching ratios of the product channels were drastically changed by the increase of E-coll. At the collision energy of 30 kcal/mol, the branching ratios of channels I:II:III were calculated to be 0.37, 0.53, and 0.10, respectively. The branching ratio for channel II became 0.69 at collision energy of 40 kcal/mol, meaning that channel II is dominant at higher collision energies, although the ratio of channel II was close to zero at thermal energy. These results suggested that the product channels in the microsolvated S(N)2 reaction are significantly affected by the collision energy. (C) 2002 Elsevier Science B.V. All rights reserved.