Wet photoelectrochemical etching has been used to create deeply etched structures in both GaAs and InP. It was observed that the conductive n+ substrates etched quite rapidly, while semi-insulating substrates exhibited enhanced etching only when a metal mask patterned the substrate, providing a low resistance contact for electron removal. It was further found that vertical profiles were achievable at high laser intensities, while a crystallographic taper was achieved at lower intensities. Finally, the limitations to microstructure geometry was explored in terms of the dynamics of the photogenerated carriers.