The composition of the adsorption layer formed on the GaAs (001) surface at the initial stages of heteroepitaxial growth of InP in a chloride vapor-transport system is calculated within a thermodynamic approach. The obtained results allow us to understand the reasons for formation of a thin GaInAsP layer of variable composition experimentally observed at the InP/GaAs interface.Heteroepitaxial A 3 B 5 /A 3 B 5 layers find wide application in modern micro-and optoelectronics, (e.g., in fabrication of fast-operating emission sources and radiation detectors [1-4]). In our earlier studies [5-8] of the initial growth stages and the properties of heteroepitaxial (001)InP/GaAs layers grown by chloride vapor phase epitaxy (VPE), we found that a thin (< 100 nm) InGaAsP intermediate layer of variable composition is spontaneously formed at the hetero-interface. An analysis of the experimental data shows that this layer results from gas etching of the GaAs substrate at the initial stage at the places, where growth centers are formed. Therefore, the components of the substrate (Ga, As) are involved in the formation of growth centers along with those of the deposited layer (In, P).In this work, we assess the composition of the adsorption layer formed on the (001) GaAs surface at the initial stages of InP heteroepitaxy in the VPE system to test the foregoing conclusion. The possibility of local gas etching of the substrate was taken into account. The calculations were made within the thermodynamic model adopted in [9, 10]. In the calculations, the growth temperature (873-1073 К) and the input pressure of phosphorus trichloride PCl 3 (100-10000 Pа) were used as variables.