The comparative catalytic activities of iron phosphides, FexP (xﾠ= 1ﾖ3), have been established with phase-pure material grown by chemical vapor deposition (CVD) from single-source organometallic precursors. This is the first report of the preparation of phase-pure thin films of FeP and Fe2P, and their identity was established with scanning-electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. All materials were deposited on fluorine-doped tin oxide (FTO) for evaluation of their activities toward the hydrogen evolution reaction (HER) of water splitting in 0.5 M H2SO4. HER activity follows the trend Fe3P > Fe2P > FeP, with Fe3P having the lowest overpotential of 49 mV at a current density of 10 mA cmﾖ2. Density functional theory (DFT) calculations are congruent with the observed activity trend with hydrogen binding favoring the iron-rich terminating surfaces of Fe3P and Fe2P over the iron-poor terminating surfaces of FeP. The results present a clear trend of activity with iron-rich phosphide phases outperforming phosphorus rich phases for hydrogen evolution. The films of Fe2P were grown using Fe(CO)4PH3ﾠ(1), while the films of FeP were prepared using either Fe(CO)4PtBuH2ﾠ(2) or the new molecule {Fe(CO)4P(H)tBu}2ﾠ(3) on quartz and FTO. Compoundﾠ3ﾠwas prepared from the reaction of PCl2tBu with a mixture of Na[HFe(CO)4] and Na2[Fe(CO)4] and characterized by single-crystal X-ray diffraction, ESI-MS, elemental analysis, andﾠ31P/1H NMR spectroscopies. Films of Fe3P were prepared as previously described from H2Fe3(CO)9PtBu (4).