The solid solubility of Zn in GaP has been determined as a function of liquid composition along the 1040°C iostherm of the ternary system. The shape of the solid solubility iostherm is similar to that obtained previously for zn in GaAs. The maximum solubility obtained was 2 sx 10 20 zn cm 3 . One significant difference between the GaAs-Zn and the GaP-Zn systems is that at high concentrations the majority of the Zn is electrically inactive in GaP after cooling from the crystal growth or diffusion temperature, while it remains electrically active in GaAs. When the loss of electrically active Zn is taken to occur by precipitation during cooling, the measured variation in solubility is described by the reaction of Zn in the liquid with Ga vacancies in the solid to form ionized Zn acceptors and free holes: Zn ɭ + V Ga ⇄ Zn Ga −+ e +. For the equilibrium calculations it was assumed that the product of the activity coefficients of Zn and P in the liquid, γ Zn ɭγ P ɭ, and that the activity coefficient of Zn on a Ga site γzn Ga, are constant in the concentration range studied. The variation of the hole activity coefficient γ h is obtained from the solid solubility isotherm and agrees with the γ h calculated from the analysis of the published data for the effective diffusion coefficient D. The Fermi level E f is given in terms of the effective density of states N ν, as p = ( N ν γ h ) exp ( E f kT ) . The analysis of the diffusion data for γ h is analogous to that previously employed for the diffusion of Zn into GaAs, where the diffusing species was taken as a singly ionized interstitial donor Zn i + whose concentration is controlled by the equilibrium: Zn i + + V Ga ⇄ Zn Ga − + 2 e +. Because the γ h , obtained in this analysis will explain two independent experiments, the concentration dependence of D and the solid solubility isotherm, strong justification of the interpretation of diffusion and solid solubility in terms of the above equilibria results.