The (photo)electrochemical and etching properties of InP and of In0.53Ga0.47As, lattice-matched to InP, in contact with alkaline OBr- solutions, are studied. Photocurrent doubling is observed at p-InP, and InP is etched very slowly. In0.53Ga0.47As is etched in a diffusion-limited way by OBr-. The reduction mechanism of OBr- at In0.53Ga0.47As depends upon the bias imposed and upon illumination. It was observed that, at In0.53Ga0.47As, the reduction of OBr- and the etching by OBr- are kinetically coupled processes. The possible use of alkaline OBr- as a material-selective etchant for In0.53Ga0.47As vs. InP is discussed.