A microcomputer based electrolyte electroreflectance (EER) system has been installed. With this system we have investigated the spatial variations of composition across the surface of Ga1-xAlxAs as well as the spatial variations of doping concentration of GaP and GaAs with a resolution of about 150 μm. We have demonstrated that EER is a convenient nondestructive tool for semiconductor characterization. The advantages of this method comparing to other techniques are discussed.