We present a theoretical study of the structural and electronic properties of CdSe/CdS and CdS/CdSe core/shell nanoparticles. We have considered relaxed structures of essentially spherical parts of the zincblende crystal structure. The electronic properties and the total energy were calculated using a parameterized density-functional tight-binding method. The results depend critically on the size of core and shell. and only in one case we find a charge separation upon excitation. We finally investigate the energetics related to the interchanqe of a S and a Se atom between core and shell. (c) 2005 Elsevier B.V. All rights reserved.