The plasma-surface interaction during CH4-H-2 reactive ion etching processing of InP is described in detail by means of plasma diagnostics (optical emission spectroscopy and mass spectrometry) and x-ray photoelectron spectroscopy (XPS) surface analysis. The influence of the input power is carried out for different CH4-H2 mixtures in terms of InP etch rate, etch product and CH3 radical detection and surface damage characterization. In particular detailed XPS results allow the study of the changes in the stoichiometry and amorphization of the surface with the input power. In addition, for a given power, the quality of the etched surface improves by increasing the fraction of methane in the gas mixture. As an example, the best surface stoichiometry (InP0.86) is obtained for a pure methane plasma running at a high power (300 W). In general, it is shown that the lower the P depletion, the lower the amorphization, which is indicative of a general improvement of the etched surface quality. Based on the XPS results, a three-layer model is proposed for the representation of the surface in the course of etching. The damaged layer situated over the bulk InP is composed of a superficial P-depleted layer and of a stoichiometric amorphized InP layer. Using the curve-fitting of the P 2p spectra, the thickness of the different layers is estimated. As an example, a damaged layer as low as 37 Angstrom thick is obtained for pure methane plasma at 15 mTorr and a power of 300 W, whereas our standard conditions (10% CH4-H-2, 50 mTorr, and 80 W) give a damaged layer of 90 Angstrom. The experimental observations give evidence of the need for both ion bombardment and active neutral species to obtain etching. The improvement of the etch process is then explained by an improved In removal rate which is actually the limiting step in the etching mechanism of InP.