The present work deals which localized dissolution processes (pit and port initiation and growth) of p- and n-type (100) GaAs. Pit and pore growth can be electrochemically initiated on both conduction types in chloride-containing solutions and leads after extended periods of time to the formation of a porous GaAs structure. In the case of p-hype material, localized dissolution is only observed if a passivating film is present on the surface, otherwise, e.g., in acidic solutions, the material suffers from a uniform attack (electropolishing) which is independent of the anion present. In contrast, localized dissolution (pitting corrosion) and pore formation on n-type material can be triggered independent of the presence of an oxide film. This is explained in terms of the different current limiting factor for the differently doped materials (oxide film in the ease of the p- and a space charge layer in the case of the n-GaAs). The porous structure was characterized by scanning electron microscopy, energy dispersive x-ray analysis, and Auger electron spectroscopy, and consists mainly of GaAs. From scratch experiments it is clear that the pit initiation process is strongly influenced by surface defects. For n-type material, atomic force microscopy investigations show that light induced roughening of the order of several hundred nanometers occurs under nonpassivating conditions. This nanometer-scale roughening however does not affect the pitting process.