Nonstoichiometric cadmium sulfide nanoparticles ([Cd2+]/[S2-] = 3) in 2-propanol were surface-modified with Cu2+ ions. Addition of copper(II) perchlorate to CdS nanoparticles leads to binding of copper ions onto the surface of the semiconductor, accompanied by rapid reduction of Cu2+ to Cu+, as confirmed by EPR and absorption spectra. Copper(II) perchlorate also quenches the recombination luminescence of CdS nanoparticles effectively. The quenching data obey a static interaction model, which confirms the binding of copper ions onto CdS. The latter was confirmed also by ultrafiltration and ICP spectroscopy. Copper ions bound onto the surface of CdS lead to formation of a new, red-shifted, luminescence band. The maximum of the new band is at 14 700 cm(-1) compared to that of the original band at 17 900 cm(-1). It is suggested that, at low copper ion concentrations, copper ions bound onto the surface of CdS nanoparticles exist as isolated Cu+ ions. They create a new energy level in the bandgap at about 1.2 eV below the conduction band, which is responsible for the new emission band (14 700 cm(-1)). Higher copper(II) perchlorate concentrations give rise to the formation of ultrasmall particles of CuxS (x = 1-2) on the surface of CdS, which eventually lead to precipitation. Both isolated Cu+ ions and ultrasmall particles of CuxS quench the original recombination luminescence of CdS nanoparticles by facilitating e(-)/h(+) nonradiative annihilation. The presence of copper ions bound onto the surface of CdS nanoparticles retard both the photocorrosion of the latter and the photodecomposition of copper(II) tetraphenylporphyrin induced by CdS nanoparticles. Appropriate mechanisms are presented.