Dry and wet chemical etching of epitaxial In0, 5Ga0.5P layers grown on GaAs substrates by gas-source molecular beam epitaxy have been investigated. For chlorine-based dry etch mixtures (PCl3/Ar or CC12F2/Ar) the etching rate of InGaP increases linearly with dc self-bias on the sample, whereas CH4/H2-based mixtures produce slower etch rates. Selectivities of ≥500 for etching GaAs over InGaP are obtained under low bias conditions with PCl3/Ar, but the surface morphologies of InGaP are rough. Both CC12F2/Ar and CH4/H2/Ar mixtures produce smooth surface morphologies and good (≥10) selectivities for etching GaAs over InGaP. The wet chemical etching rates of InGaP in H3PO4:HC1:H2O mixtures has been systemically measured as a function of etch formulation and are most rapid (∼1 μn · min−1) for high HCl compositions. The etch rate,R, in a 1:1:1 mixture is thermally activated of the formR ∝ $$e^{ - E_a /kT} $$ , whereE a = 11.25 kCal · mole−1. This is consistent with the etching being reaction-limited at the surface. This etch mixture is selective for InGaP over GaAs.