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A number of workers have shown that the vapor pressure of the reactant gases strongly influences the electrical properties of GaAs vapor grown in the ASCI system. This paper presents the results of a study of the effects of the variation of vapor pressures on GaAsP grown in the hydride synthesis system. The material was analyzed using Hall effect measurements taken as a function of temperature from room to liquid helium temperatures, and using test electroluminescent diodes. A least squares fit was made to the Hall effect data using a model which includes the effects of two valley conduction to obtain densities of donors and acceptors and donor ionization energies. Increasing reactant gas pressure causes a decrease in donor and acceptor densities as N, N∞ P -2.5 which is slower than the P observed by DiLorenzo in the ASCI system. The ionization energy of the donors was quite high (100 meV) for the high and low pressures, and were less than 10 meV for the intermediate pressures. Hall mobilities at room and liquid nitrogen temperatures were best for material grown at intermediate pressures and the electroluminescent efficiencies of unencapsulated test diodes increased from 0.05 per cent at the extremes to above 0.1-0.2 per cent at the intermediate pressures. These variations are consistent with varying Si incorporation rate and varying density of point defects with varying reactant gas vapor pressure. [ABSTRACT FROM AUTHOR]