The mechanism of the synthesis of copper(i) phosphide (Cu 3-xP) in the ionic liquid (IL) trihexyl(tetradecyl)phosphonium chloride ([P 66614][Cl]) was investigated. The phosphide formation is promoted by a transformation of red phosphorus (P red) into mobile P 4 molecules and a surface activation of copper caused by the IL including the Brønsted acidic impurity. The surface activation is important to obtain a quantitative product yield. Moreover, we demonstrate that single-phase Cu 3-xP can also be synthesized in the nitrogen-based IL tetrabutylammonium chloride ([N 4444][Cl]). Further substitution of the anion of the IL using tetrabutylammonium bromide ([N 4444][Br]) or the complete replacement of the IL by a deep eutectic solvent consisting of adipic acid and betaine do not lead to single-phase Cu 3-xP. Therefore, the nature of the anions present in the IL also seems to be relevant for the convenient phosphidization reaction.