The group III precursors trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), and triethylaluminum (TEA) have been investigated as intrinsic carbon doping sources for epilayers of GaP, Ga0.51In0.49P, and Al0.51In0.49P grown by metalorganic molecular‐beam epitaxy (MOMBE) using thermally cracked phosphine (PH3). Carbon incorporation in these films has been evaluated with calibrated secondary ion mass spectrometry (SIMS), Hall effect measurements, and capacitance‐voltage (C–V) profiling. The carbon doping properties of TMG and TEG in GaP growth are compared with those observed in MOMBE growth of GaAs. Hole concentrations in excess of 1×1020 cm-3 have been measured in films of GaP grown with TMG/PH3, which is far above that achievable in GaAs grown with TMG/AsH3 under similar growth conditions. Neither TMG nor TEG result in p‐type conductivity in films of Ga0.51In0.49P. At a growth temperature of 500 °C, SIMS measurements indicate that the carbon incorporation is inversely related to the In content of the films, with the carbon concentration falling by 1 decade for every 1.5% increase of InP mole fraction above 5%. Al0.51In0.49P films are generally highly resistive when grown with either TMA or TEA, despite incorporated atomic carbon concentrations in excess of 1×1019 cm-3. These results indicate that while carbon doping with methyl and ethyl precursors is readily extensible to GaP, carbon is not at present a useful dopant species for the MOMBE growth of In‐containing III–V phosphides due to incorporation limitations and poorly understood compensation mechanisms.