When using electroless etching of semiconductors for quality control or device applications one must be aware of the possibility of galvanic interaction in the system. Here we show how highly doped (n++) GaN in a bilayer heterostructure dramatically increases the photoetch rate of the low doped (n) GaN layer in alkaline peroxydisulphate solution (the n++ material does not etch). Contacting the bilayer to a platinum sheet in solution further increases the photoetch rate. We show how previous electrochemical studies on GaN can be used to understand such “galvanic” effects. The present results offer an explanation for unexpected features in the photoetching of compound semiconductors which, during growth, become non-uniformly doped either by accident or by design. The work describes an example of the former case: non-uniformity due to the formation of extended defects, e.g. overgrown pinholes in GaN. The relevance of this study for practical applications, including the revealing of crystallographic defects for quality control, is considered.