Photoexcited carriers redistribute the charges in the space‐charge region of a semiconductor. The wavelength‐dependent photoeffect is most significant at low temperatures and strongly affects the modulating electric field in surface barrier electroreflectance (SBER). This work undertakes a study of this effect on SBER by making concurrent ac capacitance and conductance measurements in the MOS configuration and comparing the results with photoconductivity theory. The relationship between photocapacitance and the effective modulating electric field is investigated, providing herewith a better understanding of SBER at low temperatures.