A novel and simple method is described for preparing colloidal Cu-doped ZnSe(S) quantum dots (QDs) in aqueous media by introducing copper ions using the same method as to prepare colloidal ZnSe(S). More specifically, the Cu-doped ZnSe(S) are prepared through the nucleation-doping method in the presence of 3-mercaptopropionic acid as stabilizers using zinc perchlorate, copper sulphate, and NaHSe as precursors. Confirmation of the preparation of Cu-doped ZnSe(S) nanocrystals (NCs) is done with absorption and emission spectroscopies (UV-vis and PL) as the QDs show intensive green emissions. The reduction of ions Cu2+ to Cu+ is confirmed by using electron paramagnetic resonance (EPR), in which Cu+ ions are silent. The size determination is performed by using transmission electron microscopy (TEM) and dynamic light scattering (DLS), resulting in Cu-doped ZnSe(S) particles with a mean diameter of 4.6 +/- 3.5 nm. The excellent stability observed for the nanoparticles overcomes the intrinsic instability of traditional aqueous Cu-doped ZnSe(S) NCs.