The potential energy surface of CH3F +/- H2O and SiH3F + H2O systems were scanned by B3LYP and MP2 methods. For each system two channels of reactions, i.e. with retention and inversion of configuration, were explored by modelling the front-side and the back-side attack of the water molecule. In each case the barrier heights for two channels were predicted. The inversion channels have similar and high energy barriers for both systems while the substantial difference was found for the retention channel barriers between methyl and silyl fluorides. The lower barrier for silyl fluoride is in keeping with experimental observations.