The study of microsolvation provides a deeper understanding of solvent effects on reaction dynamics. Here, the properties of the S(N)2 reaction of hydrated chloride with methyl iodide are investigated by direct dynamics simulations, and how the solute-solvent interactions and the basicity of nucleophiles can profoundly affect the atomic level dynamics is discussed in detail. The results show that the direct-rebound mechanism dominates the substitution reaction, and the roundabout mechanism, which prevails in the indirect unsolvated counterpart reaction, still accounts for a high proportion of the indirect mechanisms. The involvement of a solvent water molecule does not significantly reduce the cross section and rate constant compared to the unhydrated reaction at high collision energy. By varying solvated Cl- to F-, the dominant mechanisms are totally different and in contrast, the dynamics of water does not show much difference, and the departure of H2O tends to occur prior to the substitution reaction because of the facile breakage of the hydrogen bond at high collision energy.