A selective wet chemical etching solution for removal of Al0.5In0.5P from GaAs based on HCl:H2O has been developed. Controllable etch rates from 600 Å min−1at 1HCl:30H2O to 6000 Å min−1at 1HCl:5H2O at 25 °C are obtained. The etch rate is thermally activated of the formR∝exp(−Ea/kT) whereEa=12.4 kCal mol−1, consistent with reaction‐limited etching. Dry etch rates of AlInP in PCl3/Ar, CCl2F2/Ar, or CH4/H2/Ar electron cyclotron resonance discharges are all<300 Å min−1for additional dc self‐biases of ≤250 V. Selectivities of ≳600:1 are obtained for etching GaAs over AlInP in CCl2F2/Ar discharges for dc biases ≤150 V. The etched surface morphologies are smooth, with small quantities of Cl‐containing residues remaining after PCl3/Ar exposure, and both Cl and F detectable after CCl2F2/Ar dry etching.