Microwave modulated photoluminescence (MMPL) is a developing spectroscopy in which the sample is subjected to continuous optical pumping and chopped microwave electric fields. The signal detected in an MMPL experiment is the change in the photoluminescence spectrum of the sample due to the presence of microwave electric fields, which increase the kinetic energy of the free carriers. In order to investigate the effects of interfaces on this measured quantity, two types of GaAs samples have been compared. The first type of sample was grown such that the GaAs epilayers are exposed, while in the second type the GaAs is "capped" by a layer of higher band gap material. Several pairs of such samples have been studied. The MMPL results are consistent with the following mechanism : an increase in the kinetic energy of the free carriers that results from the presence of the microwave fields allows more free carriers to reach the interface or surface of the GaAs layer before recombining. The presence of a greater number of nonradiative recombination paths in the samples with a bare GaAs surface than at the GaAs/capping-layer interface is therefore seen experimentally as an increase in the photoluminescence quenching by the microwave electric fields. The potential usefulness of MMPL as a probe of non-radiative recombination and as an indicator of interface quality is illustrated by a quantitative estimate of surface/interface non-radiative recombination.