Electroless CoP films were deposited on NiP substrates that had been subjected to a variety of surface treatments. The interface between the CoP and NiP was then examined using Auger sputter-etch profiling, Rutherford backscattering, and secondary-ion mass spectrometry. The composition of the interface varied markedly with the NiP pretreatment. There was no detectable O incorporation in the structure when the NiP underwent chemically reducing pretreatments. A NaOH pretreatment of NiP resulted in substantial O incorporation at the CoP-NiP interface. Interfacial oxygen was generally present when there was inhibited CoP nucleation leading to magnetic films with high shape anisotropy and good magnetic properties. The source of the interfacial oxygen was the pretreatment step, rather than the initial stages of the CoP deposition. At least some of the interfacial oxygen may have been incorporated as a hydroxide. There is also mobile H in the structure, presumably arising from the electroless plating processes. © 1988, The Electrochemical Society, Inc. All rights reserved.