The microsolvatation of the S(N)2 reaction Cl-+CH3Br with one and two molecules of water has been investigated by ab initio molecular dynamics. The energy profile and the stationary points have been discussed. For the case of one molecule of water the thermal effects have been evaluated and some impact trajectories have been simulated. The results, in agreement with recent experimental results, suggest the noneffectiveness of the S(N)2 mechanism in the presence of solvatation. The analysis in terms of localized electron density has shown the importance of the H-bond between the chloride ion and the water molecule during the reaction path. (C) 2001 American Institute of Physics.