This study investigated the reductive de-halogenations of toxic Br3CCO2-, Br2CHCO2-, BrCH2CO2-, CH3CHBrCO2-, CH2BiCH2CO2-, CH2BrCHBrCO2-, CI3CCO2-, Cl2CHCO2- and C1CH2CO2- by sodium borohydride catalyzed by sol-gel silica entrapped Au-0 and Ag-0 nanoparticles. The results indicate that the mechanism of reduction of Br3CCO2- differs from that of Cl3CCO2-. Calculated by DFT, the source of this difference lies in the larger bond strength of C-Cl compared to that of C-Br and the weaker M-0-C bond strength in Au-0-CBr2CO2- compared to those of Au-0-CCl2CO2- and Au-0-CH2CO2-. Furthermore, the de-halogenation mechanisms catalyzed by Ag-0-NPs differ from those catalyzed by Au-0-NPs. The latter observation is attributed to the different Ag-C and Au-C bond strengths and to the different over-potentials for H-2 release of these M-0-NPs. In addition, product composition depends on the rate of BH4- addition. Proton labeling experiments prove that nearly all the hydrogen atoms in the products originated from the water solvent and not from the BH4-. The detailed mechanistic conclusions that can be drawn from these results differ considerably from those commonly accepted for de-halogenation processes.