We developed a three-step colloidal synthesis of near-infrared active Cu-In-Se (CISe)/ZnS core/shell nanocrystals (NCs) via a sequential partial cation exchange. In the first step binary highly copper deficient Cu2‒xSe NCs were synthesized, followed by a partial cation exchange of copper to indium ions yielding CISe NCs. In order to enhance the stability and the photoluminescence (PL) properties of the NCs, a subsequent ZnS shell was grown, resulting in CISe/ZnS core/shell NCs. These core/shell hetero-NCs exhibited a dramatic increase in size and a restructuring to trigonal pyramidal particles. The reaction parameters, e.g. the Cu:Se-ratio, the temperature and the time were carefully tuned enabling a distinct control over the size and the composition of the NCs. By varying only the size of the CISe/ZnS NCs (from 9 to 18 nm) the PL spectra could be tuned covering a wide range with maxima from 990 nm to 1210 nm. Thus, in these experiments we demonstrate a clear dependence of the optical properties of these materials on their size and extend the PL range of CISe-based nanoparticles further to the infrared part of the spectrum. Furthermore, the relatively large size of these NCs allows their detailed structural analysis via electron microscopy techniques, which is particularly challenging in the case of small particles and especially important to relate the size, composition and crystal structure to their optoelectronic properties.