Tri-n-octylphosphine (TOP) is widely used as a phosphorating agent to yield metal phosphide nanocatalysts, which are receiving intense attention because of their high performance in both electrochemical and hydrotreatment catalytic processes, such as the hydrogen evolution reaction and the hydrodesulfurization reaction. Using in situ ambient-pressure X-ray photoelectron spectroscopy, the formation of nickel phosphide on the surface of a nickel foil was investigated, at temperatures like those employed to form nickel phosphide nanoparticles in the colloidal route. We uncovered that the onset of phosphorus insertion into nickel is as low as 150 °C, much lower than reported in the literature 2 (> 210 °C). Moreover, formation of sp2 carbon was observed on the surface as the consequence of TOP alkyl chain decomposition. These findings provide new insight on the surface chemistry of metal phosphide nanoparticles, increasingly employed in several fields of catalysis. Our results demonstrate that even below 150 °C, significant phosphorus and carbon incorporation can occur during metal nanoparticles syntheses that employ TOP as stabilizing agent.