The strict virtual crystal model has been shown to describe accurately the mixing enthalpy of III-V ternary alloys. We allocate the different components of this model to both subreactions, i.e., the reactions forming III-V-I and III-V-II, present in the molecular-beam epitaxial growth of III-(VVII)-V-I compound alloys, and derive thereof a model that describes accurately the composition obtained. A good correspondence with experimental growth data is obtained. We apply this model to both relaxed and strained layer growth and the evolution in the composition is given when a relaxed layer is grown on a substrate with a different lattice constant.