A process for reactive ion etching of GaAs and InP is described using a mixture of ClCH₃/X, where X=H₂, He, O₂, Ne or Ar. The ClCH₃/H₂ process is shown to etch GaAs and InP with anisotropic etch rates of up to 80 and 300 nm min^-1, respectively. For the ClCH₃/O₂ process an etch rate of 11 nm min^-1 is obtained for GaAs with no appreciable etching of InP. This simple combination of gases leads to the realization of selective InP over GaAs or GaAs over InP etching. When compared with CH₄/H₂ RF plasma etching in the same reactor, a significant reduction in the measured applied RF power density and self-bias is observed.