Electron cyclotron resonance plasma oxidation of InP was studied using both spectroscopic and single wavelength ellipsometry employed during the oxidation process. A two layer oxide was observed with the outer layer being In rich and the inner layer P rich as confirmed from x‐ray photoelectron spectroscopy and etch rate studies. Optical models and oxidation kinetics are analyzed. From positive substrate bias effects on oxidation rates, negative ion oxidant species were identified as dominant and oxide etching was observed at negative bias. Plasma oxidation, like thermal oxidation, yielded excess P near the semiconductor surface which would degrade electronic properties.