We report the results of infrared photoluminescence and optical transmission measurements of InAs epilayers grown on both Si and GaAs substrates. Photoluminescence reveals four well-defined peaks associated with band-to-band transitions and impurity/defect associated transitions. The spectra of the epilayers grown on Si are red shifted by roughly 15 meV relative to that of the epilayers grown on GaAs. In addition, optical transmission measurements reveal two intrinsic absorption edges for the epilayers grown on Si. These data are interpreted in terms of biaxial strain induced between the epilayer and the substrate due to differing indices of linear thermal expansion.