We have studied the photoreflectance (PR) spectra from a molecular‐beam epitaxially (MBE) grown heterostructure consisting of 200 nm of Ga0.83Al0.17As , a 800‐nm GaAs buffer layer on a semi‐insulating (SI) 〈100〉 (LEC) GaAs substrate. By varying both the pump beam wavelength and modulation frequency we are able to identify the component layers, their quality, and the quality of the various interfaces. In this study we find evidence for a low density of interface states between the GaAs buffer layer and the GaAlAs layer, and a relatively large density of interface states, between the substrate and buffer layers. These states, previously observed by deep‐level transient spectroscopy (DLTS) of doped structures, are presumably associated with the interface produced by MBE growth on etched and air exposed substrates. However, in our experiment since the substrate is semi‐insulating and the buffer layer is undoped, it is difficult to resolve these states spatially byC–Vtechniques. Our results show that the PR technique can be used to characterize low conductivity or semi‐insulating structures such as enhancement mode metal–semiconductor field effect transistor (MESFET) and high electron mobility transistor (HEMT) type devices and it may be useful for theinsitucharacterization of epigrown surfaces and interfaces.