ZnSe layers have been grown by molecular beam epitaxy under similar conditions on both (100) GaAs and (100) Ge substrates. The GaAs and Ge substrates were prepared in an identical fashion which involged a process of argon-ion sputtering at room temperature followed by annealing at ~400 °. The surface reconstructions observed following the in-situ cleaning process were (4x1) and (2x2) for the GaAs and Ge substrates, respectively. ZnSe layers were grown using a unity beam pressure ratio at a substrate temperature of 330°C to various thicknesses in the range 1.0 to 6.5 μm. The quality of the ZnSe/GaAs and ZnSe/Ge layers was compared by measuring the linewidths of double-crystal rocking curve (DCRC) peaks and donor-bound exciton (DBE) peaks obtained from the layers by X-ray diffractometry and (4.2K) photoluminescence measurements, respectively. The X-ray studies revealed the ZnSe/Ge layers and to a lesser extent the ZnSe/GaAs layers to be tilted with respect to the substrate orientation. Typical tilt angles recorded were ~1,000" and ~40" for ZnSe/Ge and ZnSe/GaAs layers, respectively.