In order to optimize the H-2/CH4 reactive ion etching process for InP, the surface composition of etched samples has been analyzed by means of a focused ion beam and secondary ion mass spectrometry. Harmful species of chlorine and oxygen have been identified as causing surface roughness and etch-rate fluctuations. polymer distribution on the masked surface and etched surface phosphor abundance have been studied for various gas mixtures and rf power, establishing optimized CH4-rich etching conditions under which surface P content is closest to unetched material and polymer deposition is acceptable.