Optical emission spectroscopy (OES) has been used in order to investigate the InP etching mechanisms in a Cl-2-H-2 inductively coupled plasma. The authors have previously shown that anisotropic etching of InP could be achieved for a H-2 percentage in the 35%-45% range where the InP etch rate also presents a local maximum [J. Vac. Sci. Technol. B 24, 2381 (2006)], and that anisotropic etching was due to an enhanced passivation of the etched sidewalls by a silicon oxide layer [J. Vac. Sci. Technol. B 26, 666 (2008)]. In this work, it is shown that this etching behavior is related to a maximum in the H atom concentration in the plasma. The possible enhancement of the sidewall passivation process in the presence of H is investigated by comparing OES measurements and etching results obtained for Cl-2-H-2 and Cl-2-Ar gas mixtures.