The surface photovoltage (SPV) effect induced by the defect states in semi-insulating (SI) GaAs was studied. The PV response below the band edge was measured at room temperature with a dc optical biasing. The spectra were found to be strongly dependent on the surface recombination and were attributed to formation of the carrier concentration gradient near the surface region, showing that SPV is a very sensitive and nondestructive technique for characterizing the surface quality of the SI-GaAs wafers.