Reactive ion etching of InP with CH4/H2mixtures, a promising process for optoelectronic device fabrication, has been studied to understand the mechanisms of etching and anisotropy. Special attention has been paid to the polymer film that deposits on inert surfaces in the discharge; deposition rates have been used as a monitor of the discharge chemistry as well as for process optimization. Surface analysis shows that under etching conditions that maximize the InP etch rate while minimizing polymer deposition, the hydrocarbon coverage on the InP surface equals typical ‘‘adventitious’’ carbon levels, and the surface is significantly depleted of P. The etch rate here is limited by the flux to the surface of hydrocarbon reactants responsible for In desorption. The absence of a significant hydrocarbon film on the vertical‐etched surfaces under conditions of 8:1 anisotropy precludes a surface inhibitor mechanism of anisotropy, implicating instead energy deposition via ion bombardment as the major contributor to the enhanced vertical etch rate. As the feedstock methane fraction is increased, more stoichiometric surfaces are obtained, the polymer deposition rate and the abundance of gas phase hydrocarbon oligomers increases, and ultimately, polymer forms on the InP. Here the InP etch rate is limited by transport through the permeable polymer overlayer. Reactions with polymer‐coated chamber walls are important in determining InP etch and polymer deposition rates, illustrating the need for chamber seasoning to obtain reproducible results. PH3is identified by mass spectrometry as the primary P‐containing volatile product, while the primary In‐containing volatile product remains unidentified.