The reaction mechanism describing the formation and surface decomposition of Ni3P has been investigated. Ni3P was synthesized as a thick film on a Ni foil by a simple solid-state reaction. X-ray diffraction confirms that the layer produced, on the surface of the Ni foil, is pure Ni3P. The solid-state reaction did not occur at 400° C or lower temperatures. At 600° C or higher temperatures the solid-state reaction occurred by a nucleation mechanism of synthesis and later sintering of Ni3P particles at the surface of the Ni foil. The surface of the Ni3P-Ni foil showed to be stable in air up to 300° C. At higher temperatures, Ni3P decomposed trough a complex mechanism, including the loss of phosphorus, the formation of the phases Ni12P5 and Ni2P, and finally, oxidation of the nickel phosphides. Samples heated at 800 ° C for 4 h produced a NiO layer on the surface of the Ni3P-Ni foil. © 2005 Elsevier B.V. All rights reserved.