A technique to etch GaP by reactive ion etching was developed and the effects of different etching parameters were studied. Also, selective etching of GaP over AlGaP was examined and demonstrated. Etching is achieved by using SiCl4, which will react with GaP to form volatile compounds. Selective etching is accomplished when SiF4 is used in addition to SiCl4. The addition of the fluorine-based gas will result in a nonvolatile etch-inhibiting layer, AlF3, when aluminum is present on the sample surface. By adjusting etching parameters, a selectivity as high as 126 is demonstrated. The presence of the AlF3 etch-inhibiting layer is verified by Auger electron spectroscopy, and the removal of this layer by buffered oxide etch is demonstrated. In addition, a direct comparison of etch rates for GaP and GaAs was made, and etch rates were found to be similar.