Galvanic cell formation during dissolution of semiconductor p/n structures in electroless and chemical etchants was studied with GaAs as a model system. Experiments were performed in a two-compartment cell in which etching of the p- and n-sides of the p/n sample could be investigated separately. The two sides could also be exposed to solutions of different composition and to different etching conditions. In particular, the role of illumination in galvanic cell formation was considered. Energy band diagrams and mixed-potential theory are used to explain and predict the etching properties of the p/n system.