In this article we show in situ and ex situ experimental reflectance difference spectroscopy (RDS) data of ZnSe(001) grown on GaAs, where special attention is focused on the resonancelike structure at the band gap of ZnSe. This feature was reported previously and interpreted as either the transition from the valence band of ZnSe to a quantum well state at the ZnSe/GaAs interface or it was attributed to interface states. We report a new interpretation of this spectral feature which considers anisotropic strain in the ZnSe/GaAs heterostructures. By straining the ZnSe(001) epilayers mechanically, it was observed that the resonance feature changed sign and magnitude, proving that the height of this in-plane anisotropy feature is a measure for the in-plane strain. In-plane strain induces a splitting and shift in energy of the light and heavy hole; valence bands. The transition between the light hole valence band and the conduction band is not polarization dependent, however the transition between the heavy hole valence band and the conduction band, as well as the transition between the spin split-off band and the conduction band shows polarization dependence. This difference between the reflectances parallel and perpendicular to the in-plane strain direction can be measured ex situ and in situ with RDS.