Thermodynamic calculations are presented which provide explicit expressions for the combined temperature and phosphorus partial pressure dependences of the Ga- and P-vacancy concentrations in GaP. This work extends the applicability of earlier results for the vacancy concentrations, obtained by an analysis of the solidus data, from solid-liquid (i.e., LPE and LEC growth) to solid-vapor equilibria (vapor growth, annealing, and diffusion). The calculations are represented in the form of Ga- and P-vacancy concentration and solid-composition isobars. In addition, the previously established correlation between the reciprocal of the nonradiative shunt-path lifetime (1/τn) and the relative Ga-vacancy concentration for GaP crystals prepared by a variety of techniques is transformed into a correlation with absolute vacancy concentrations. The electron-capture cross section of a Ga-vacancy behaving as a shunt path in p -type material is estimated to be ∼4.5×10−17 cm2. Finally, the Ga-vacancy concentration isobars and the 1/τn correlation are discussed in the context of LPE, vapor growth, and annealing processes for GaP.