To get insight into the selective etching of Al0.25Ga0.75As with respect to GaAs in acidic I-2 + I- containing solutions, etch rate measurements on the (100)-face of both materials were performed as a function of several variables. By combining these results with those of electrochemical measurements (including rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques), it was established that at Al0.25Ga0.75As an electroless etching mechanism operates, whereas at GaAs dissolution occurs by chemical etching. The observed selectivity in solutions with I-2 concentrations above 0.01 mol . liter(-1) is essentially due to this difference in etching mechanism, which determines the concentration dependence of the etch rate as well as the influence of surface oxide layers on these rates.