Potentiostatic experiments in KOH solution were used to investigate the photoetching of the Ga-polar face of heteroepitaxial GaN layers grown on sapphire. Different etching regimes are identified; these depend on the applied potential, KOH concentration, light intensity and electron concentration. In particular, the importance of the relative rates of transport of photogenerated holes and OH− ions to the surface for the etching kinetics and morphology is demonstrated. Consequently, the hydrodynamics of the etching system are important. These results form the basis for a comparison with a more widely used approach: photoenhanced open-circuit etching with a counter electrode.