Nominally binary CdSe quantum wells (QWs) with thicknesses From 1.0 to 2.5 monolayers embedded in ZnSe were grown by molecular beam epitaxy on GaAs(001) substrates. The investigation of the samples by high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) is focused on the vertical cadmium distribution in the QW region. Both the total Cd content and the real QW thicknesses determined by HRXRD and HRTEM are in good agreement. The findings point to the formation of ternary ZnCdSe wells with thicknesses of 4-6 monolayers nearly independent of the intended QW thickness and applied substrate temperature during growth. This can be ascribed either to a strongly enhanced Cd diffusion in ZnSe or to Cd surface segregation.