We have used an effusion cell with independently valved solid arsenic and red phosphorus sources and a common cracking zone to prepare extremely uniform composition GaAs(1-x)P-x epitaxial films on 3 in. diam substrates. Mass spectrometry and ion gauge measurements of dimer species allowed determination of ionization sensitivities for phosphorus and arsenic species in good agreement with the relationship of Flaim and Ownby [J. Vac. Sci. Technol. 8, 661 (1971)]. From the mass spectrometer data, it was possible to determine the thermodynamic equilibrium constant 1.9+/-0.4, and hence free energy -0.55 eV molecule(-1) for the process As-2+P-2 double left right arrow 2 AsP. Finally double heterojunction GaInAsP/InP lasers grown with this cell and operating at room temperature have transparency threshold currents below 800 A cm(-2).