The reduction mechanism of hypophosphorous acid has been investigated by ab initio calculations at the MP2/6-311G(d,p) and CBS-QB3 levels. Two mechanisms with three possible pathways have been proposed for the process. The first mechanism that can be viewed as a direct mechanism contains two possible pathways, giving H3PO2+H -> P+2H(2)O. The other, as an indirect mechanism, is H3PO2+2H -> PH3+2OH(-). Theoretical calculations predict four transition states with the highest barrier height of 37.7 kcal/mol in the indirect mechanism, which is the most likely pathway compared to others.