The reduction of nitromethane has been studied by impedance voltammetry at various water activities using NaClO4 as the base electrolyte. The impedance analysis conforms well to an EC-C-eee mechanism, and the rate constants for the three rate determining steps are obtained together with the standard potential for the first reduction process (EC). On the other hand, the results obtained for the reduction of deuterated nitromethane in 1 M NaCl + D2O solutions are compared with the ones obtained for nitromethane in 1 M NaCl aqueous solutions. A special role played by the solvent on the first chemical step shows up in both kinds of experiments, thus suggesting that it consists of a nitro-aci tautomerism reaction.