The Cahn-Hillard equation, previously derived to describe the decomposition process of metallic substitution solid solutions, is adapted to consider the spinodal decomposition process of semiconductor solid solutions. The obtained differential equation for the decomposition of a material is used to describe the composition modulation effect observed during the synthesis of GaxIn1-xP-GaAs heterostructures. These heterocompositions are used as active media of modern optoelectronic devices. The numerical simulation of the process of spinodal decomposition of the GaxIn1-xP solid solution is performed. The intervals of the thermodynamic parameters for the technological process of the structures synthesis, in which the composition modulation effect should appear, are found. The results of quantitative analysis are compared with experimental data.