We have fabricated a series of ZnSe/ZnCdSe multiple-quantum-well (MQW) structures in order to test the dependence of the resultant index of refraction n of the composite system on the n values of the constituents. The ZnSe/ZnCdSe MQWs were grown by molecular-beam epitaxy on GaAs substrates. We used photoluminescence and x-ray diffraction experiments to initially characterize the specimens. Using a prism coupler system-which measures n with high precision-we obtained n at three discrete laser wavelengths. We find that the effective values of n of these MQWs differ from their "Vegard-law-like" values; that is, the n of the composite structure is nota linear function of the n values of the constituents. This departure results from the energy-level structure of the MQW system associated with confinement of electrons and holes in the well layers.