A method is provided for determining the minority carrier surface recombination lifetime constant (t.sub.s) of a specimen of semiconductor material. The specimen is positioned between a pair of electrodes, the specimen being disposed on one of the electrodes and being spaced form the other electrode. A signal is provided corresponding to the capacitance between the specimen and the electrode spaced from the specimen. A region of the surface of the specimen is illuminated with a beam of light of predetermined wavelengths and which is intensity modulated at a predetermined frequency and varying in intensity over a predetermined range. A fixed bias voltage V.sub.g applied between the pair of electrodes, the fixed bias voltage being of a value such that the semiconductor surface is in a state of depletion or inversion,. A signal is provided representing the ac photocurrent induced at the region of the specimen illuminated by the light beam. The intensity of the light beam and frequency of modulation of the light beam are selected such that the ac photocurrent is nearly proportional to the intensity of the light beam and reciprocally proportional to the frequency of modulation of the light beam. A signal is provided corresponding to the illumination intensity of the beam of light. The surface minority carrier recombination time constant (t.sub.s) is then determined using the ac photocurrent capacitance and illumination intensity information.