Microwave modulated photoluminescence (MMPL) is a characterization technique in which a semiconducting sample is subjected to continuous optical pumping and chopped microwave electric fields. The signal normally detected in an MMPL experiment is the change in the photoluminescence (PL) spectrum due to the presence of the microwave electric field, which increases the kinetic energy of the free carriers. We have previously correlated the quenching of the PL signal, as induced by the microwaves, with nonradiative recombination at a surface/interface of the photoexcited volume. In this work, we determine quantitatively surface recombination velocities through a combined measurement of microwave induced changes in photoconductivity and in FL. From the change in the photoconductivity we infer a change in the diffusion constant of free carriers in the material. The change in diffusion constant, along with the change in luminescent intensity, uniquely determines the surface recombination velocity of the layer. Results for GaAs layers with bare surfaces are presented and the potential usefulness of the technique to other material systems, including the measurement of properties of buried interfaces, is discussed.