Abstract To understand the basic processes of altered layer formation in subsurface regions of an alloy, we studied the dynamic change of the composition profile in the altered layer by Monte Carlo simulation. The simulation was performed for Cu 52Pt alloy under 3 keV Ar + ion beam irradiation at an incident angle of 45°. Model of the simulation is based on the descriptions of three processes; cascade mixing, radiation-enhanced Gibbsian segregation (REGS) and radiation-enhanced diffusion (RED). In the simulation we used the experimental results for assessing the composition of the 1st atomic layer and of different values of the RED coefficient. The results have revealed that the coefficient D ≈ 3 × 10 −16cm 2/s is most appropriate, satisfying the mass balance law at steady state attained after an experimentally determined ion dose. Preferential sputtering of Cu atoms is confirmed in the initial stage of the sputtering, and the composition ratio of sputtered Cu and Pt atoms approaches that of the bulk as sputtering proceeds.