We used wet chemical methods to synthesize core–shell nanocrystalline samples CdS(d)/ZnSe N , where d = 3–6 nm and N = 1–5 are the size of CdS cores and the number of monolayers grown on the cores, respectively. By annealing typical CdS(d)/ZnSe N samples (with d = 3 and 6 nm and N = 2) at 300 °C for various times t an = 10–600 min, we created an intermediate layer composed of Zn1−x Cd x Se and Cd1−x Zn x S alloys with various thicknesses. The formation of core–shell structures and intermediate layers was monitored by Raman scattering and UV–vis absorption spectrometers. Careful photoluminescence studies revealed that the as-prepared CdS(d)/ZnSe N samples with d = 5 nm and N = 2–4, and the annealed samples CdS(3 nm)/ZnSe2 with t an ≤ 60 min and CdS(6 nm)/ZnSe2 with t an ≤ 180 min, show the emission characteristics of type-II systems. Meanwhile, the other samples show the emission characteristics of type-I systems. These results prove that the partial separation of photoexcited carriers between the core and shell is dependent strongly on the engineered core–shell nanostructures, meaning the sizes of the core, shell, and intermediate layers. With the tunable luminescence properties, CdS–ZnSe-based core–shell materials are considered as promising candidates for multiple-exciton generation and single-photon sources.