Codeposition of phosphorus with nickel on an Ni-Ag alloy electrode has been investigated by means of in situ surface enhanced Raman spectroscopy (SERS) to obtain information about the phosphorus incorporation mechanism during electrodeposition of Ni-P alloys. The experimental results showed that in the solution without NiSO4, hypophosphite was reduced only to a Ni-phosphine compound, while in the case where NiSO4, coexisted in the solution, the Ni-phosphine compound, as an intermediate, was oxidized by Ni2+ to elemental phosphorus in alloys with nickel acting as the catalyst.