Tribofilms, formed by tribochemical reactions between steel surface and trimethylphosphite in gas phase, have been characterized by various structural and analytical techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). Focus ion beam technique has been used to prepare cross-sections of the tribofilm for further TEM and EDS analyses. Cross-sectional views of tribofilms obtained by gas phase lubrication show that tribofilm has an inhomogeneous thickness ranging from 10 to 50 nm and is amorphous. Complementary XPS and EDS analyses confirm the formation of various iron phosphides as the major component of the tribofilms, with a preferential nucleation of Fe3P stoichiometry at the interface between steel and tribofilm. (C) 2012 Elsevier B. V. All rights reserved.