The thermal dissociation of PH3 and NH3 injected through a Ta‐based high‐pressure gas‐injection cell were studied. Using a quadrupole mass spectrometer, the Ta was found to effectively dissociate the PH3 into primarily P2, P4, and H2 molecules with an injector temperature as low as 550 °C. The introduction of NH3 through this same injector resulted in dissociation into N2 and H2 for an injector temperature greater than 900 °C. When NH3 and PH3 were coinjected, again P2, P4, N2, and H2 were the primary molecules produced; however, an additional dimer, PN, was also detected. The PN concentration was maximum at a cell temperature of 700 °C. This dimer was found to be an effective source for introducing nitrogen as an isoelectronic trap in GaP.