We have investigated the behaviour of n- and p-doped GaAs(111)A electrodes in sulfuric acid solution by electrochemical impedance spectroscopy (EIS) and second harmonic generation (SHG) measurements. The potential dependence of the SHG response was found to be closely related to the changes in the surface state population, as revealed by analysis of the impedance data. The nature of the majority of carriers turned out to be a key factor in shaping the surface state- and field-effect on the second harmonic generation process.