Ternary and quaternary III-V alloys are important for many optical device applications, and a precise control of the composition is required. Molecular beam epitaxy (MBE) is generally considered a non-equilibrium or kinetically controlled process but most of these models are too computationally intensive for real time control. We report on using a precursor state growth model 1,2 for the growth of GaAsSb to control the growth conditions and hence the film composition. The activation energies and the parameters appearing in the relationship are determined by fitting the calculated compositions to experimental ones as determined by x-ray diffraction. The effect of substrate temperature, growth rate and flux intensities on composition is discussed.