Etching experiments were carried out on partially masked GaAs single crystals in alkaline K3Fe(CN)6 solutions inwhich the dissolution rate of all crystal planes is diffusion-controlled. Etching could be rate determined in two ways. Inthe first case, mass transport of OH– ions to the GaAs surface determined the rate of the anodic partial reaction and alsothe etch rate. This resulted in rounded profiles at the semiconductor/resist edge as expected for diffusion limited etching.In the second case, mass transport limited reduction of the oxidizing agent determined the dissolution rate. Etching at theresist edge was now, surprisingly, anisotropic and faceted profiles were observed. On the basis of electrochemical measurementswith these etchants it is concluded that a local galvanic element can be formed between crystallographic facets.As a result, certain facets may be cathodically protected and consequently etch more slowly than the corresponding freecrystal plane.