Thin films of Ni3P ca. 400nm in thickness were obtained by electrodeposition on a stainless steel substrate and subsequent heating at 500°C in an argon or vacuum atmosphere. The films were found to consist of pseudo-spherical agglomerates of nanosized particles (less than 70nm in size). Both types of atmospheres induced the formation of coating layers composed of metal oxide species (particularly iron oxide) by effect of the presence of oxygen traces in the environment. As revealed by the XPS depth profile analysis, the layers were thicker when heating under argon. Film thickness determined the electrochemical reactivity towards lithium. Oxide and electrolyte reduction were the main electrochemical reactions undergone by the film on discharging the cell, the Ni–P alloy component being scarcely affected. Based on XRD, XPS and electrochemical measurements, the pristine film heated under vacuum also reacted partially with lithium to give nickel and lithium phosphide. However, the reversibility of this reaction is limited and the capacity delivered by the resulting cells was low (lower than that calculated from the reaction stoichiometry, but significantly higher than that obtained from the electrode made from the bulk alloy).