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Various reports on phosphorus‐doped diamond growth present a prominent variation in the doping profile and the doping gradient at the substrate/epilayer interface. This warrants a closer investigation of the growth process, in particular, the gas chemistry via residual gas analysis (RGA) to determine whether a doping indicator exists that would allow a real‐time control of the phosphorus incorporation. Phosphorus‐doped diamond films are prepared by plasma‐enhanced chemical vapor deposition utilizing a 200 ppm trimethylphosphine in hydrogen gas mixture. The phosphorus‐doped diamond growth is characterized by in situ RGA, which identifies a diatomic radical (PH) formed in the hydrogen plasma. A rapid analysis response is achieved through an engineered differentially pumped component. Secondary ion mass spectroscopy (SIMS) is employed to evaluate the phosphorus incorporation in the doped diamond epilayers. The SIMS‐derived phosphorus doping profile is correlated to the RGA‐measured PH concentration. For an epilayer grown on a (111) chemical vapor deposition‐type IIa substrate with moderate miscut a significant phosphorus incorporation of 4.5 × 10¹⁹ cm⁻³is measured with an incorporation efficiency of about 10%. A doping model is derived that utilizes RGA for dominant growth and doping species and under consideration of various growth modes.