The influence of the CH4-H2mixture composition on the etching process of InP is investigated by means of plasma diagnostics (optical emission spectroscopy and mass spectrometry) and quasiin situx-ray photoelectron spectroscopy (XPS) surface analysis. Increasing the mixture composition in methane increases the InP etch rate. For example, it rises from 230 to 380 Å/min when increasing the percentage in CH4from 2.5% to 75%. In pure methane discharge, at a pressure of 50 mTorr, amorphous carbon is deposited on InP. Quasiin situXPS reveals major changes in the surface chemistry. In particular P depletion decreases and the mean surface stoichiometry improves as the percentage of methane increases. The mass spectrometry PH3+signal(m/e=34 amu) corresponding to PH3molecules and the In*emission line(λ=451.1 nm), which are, respectively, characteristic signals of the individual removal rate of In and P, and the concentration of CH3radicals in the plasma as measured by the threshold ionization technique display good agreement with the etch rate and surface chemical modifications. These results confirm that the etching mechanism of InP is controlled by the etching mechanism of In, and that the latter is strongly correlated with the concentration of methane in the mixture. Variable photoelectron emission angle measurements are performed to determine precisely the location of the species in the damaged layer. A model for the representation of the surface in the process of etching is then proposed.