The electrochemical and etching behaviour of n- and p-(100)GaAs in aqueous iodic acid solutions (pH 0) was studied by rotating-(ring)-disk voltammetry, impedance measurements and etch rate measurements. Under all circumstances, a competition for HIO3 exists between electrochemical and etching reactions, the supply of HIO3 to the surface being diffusion-controlled. A particularly interesting effect is the cathodic photocurrent enhancement, up to a factor of about 6, which is observed at the p-GaAs electrode. ?The experimental results allow us to propose mechanisms for the various processes under investigation. Differences in electrochemical and etching behaviour compared with the systems GaAs/IO3- (pH 14) and InP/HIO3 (pH 0) are discussed.