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We have studied the photoreflectance (PR) spectra from a MBE grown heterostructure consisting of 200 nm of Gao.s3A10.~7As, a 800 nm GaAs buffer layer on a semi-insulating (100) LEC GaAs substrate. By varying both the pump beam wavelength and modulation frequency (up to 100 kHz) we are able to identify the component layers, their quality and the properties of the various interfaces. In this study we find evidence for a low density of interface states between the GaAs buffer layer and GaA1As layer and a rel-atively large density of interface states between the substrate and buffer regions. These states, previously observed by Deep Level Transient Spectroscopy of doped structures, are presumably associated with the interface produced by MBE growth on etched and air exposed substrates. However, in our material, since the substrate is semi-insulating and the buffer layer is undoped, it is difficult to resolve these states spatially by C-V techniques. Our results show that the PR technique can be used to characterize low conductivity or semi-insulating structures uch as enhancement mode MESFET and HEMT type devices and it may be useful for the in-situ characterization f epigrown surfaces and interface