This work presents the fabrication of type-II CdS/ZnSe core/shell heterostructures based on the wet chemical method in octadecene (ODE) solvent. The fabrication processes were monitored by spectrometric techniques of UV–vis absorption, photoluminescence (PL) and Raman scattering (RS). We point out that CdS cores are necessary to be grown at high temperatures of 290–310 °C to improve their crystal quality. Meanwhile, the ZnSe-shell growth is needed to be carried out at low-enough temperatures, and the injection speed of the precursors CdS cores, and Zn2+ and Se2− ions into the reaction solvent is as swift as possible. These conditions are necessary to restrict the dissolution of CdS cores during the ZnSe shell growth, and the formation of the other structure types, such as CdS/CdSe or CdS/(Cd,Zn)–(S,Se)/ZnSe. We also suggest typical signatures to identify the successful fabrication of type-II CdS/ZnSe core/shell heterostructures, which are based on the tail features of UV–vis absorption spectra, the linear dependence of the PL-peak energy on a cubic root of the excitation power, and long lifetimes of emission peaks. Besides these signatures, the study of phonon-vibration spectra is also necessary. Having studied the dependences of emission energy on excitation energy, we found an optimal thickness of the ZnSe shell coated on CdS cores. Apart from this thickness, the spatial separation of the carriers between the CdS core and the ZnSe shell will be significantly declined.