The atomic concentration ratio of In to P on InP(100) surfaces has been determined to an accuracy of 20% by measuring the In 3d5/2 and P 2p XPS intensities and using the corresponding atomic sensitivity factor values previously derived by the authors. The experimental results show that there exists a preferential sputtering effect for the argon ion bombardment. The In to P concentration ratio reaches a value of 2.2 at 1 keV ion beam energy and increases monotonically with the ion energy. After annealing the sample at approximate 300℃ in ultra high vacuum for 30 minutes, the In to P ratio at the surface decreases to 1.2 which is quite closed to the value of bulk stoichiometric ratio. It can be deduced from the angular dependence of the XPS intensities and the chemical shift of In 3d5/2 binding energy that after the heat treatment, the In-P chemical bonds which were broken by the ion bombarding could reconnect, and the surface atomic arrangement was thus led to become an ordered structure with an outmost atomic layer rich in In.