Low energy ion bombardment can induce compositional changes in the surfaces of compound materials. A fundamental problem is to determine which of the two main mechanisms caused the compositional change, viz. preferential sputtering or bombardment-induced segregation. This paper describes a method, using Auger electron spectroscopy (AES) taken at different angles, to determine the real (top) surface concentrations for an InP (1 0 0) surface after 5 keV Ar+ bombardment at varying impact angles. This bombardment results in an altered near-surface layer. This altered surface layer is amorphised and has a non-stoichiometric surface composition. AES intensity measures the average concentration over the information depth. In this paper, two deconvolution models were used to determine concentration vs depth distributions from the AES intensities. These two models were then used to calculate a surface concentration for each case. Using a deconvolution model in which chemical effects and segregation dominate, the calculated surface concentration was larger than 1, indicating an unphysical surface concentration. Applying a ballistic deconvolution model in the quantification equation, the surface concentration values determined, agree within 5% to the values obtained from TRIDYN simulations. From this follows that argon ion bombardment-induced compositional changes in InP are mainly due to preferential sputtering and ion beam mixing and (to a lesser extent) bombardment-induced diffusion.