This work is devoted to a photoreflectance (PR) study of single GaAsSb layers grown pseudomorphically on InP. Such antimonide alloys included in the base of heterojunction bipolar transistors (HBT) are expected to lead to high cut-off frequency transistors. However, many important parameters such as the nature of the InP/GaAsSb interface, the conduction band offset and the Fermi level pinning at GaAsSb surface are not well known in this system. In this study, they are determined from optical measurements performed between 8 and 300 K. Unusual evolution of the photovoltage as a function of temperature is reported in GaAsSb. Finally, a PR study of a complete HBT structure shows the potential of this technique to non-destructively characterize complex epitaxial layers, and further on to study fully processed devices under operation. (C) 2003 Elsevier B.V. All rights reserved.