A new colloidal procedure, for the synthesis of PbSe and PbSe/PbS core-shell semiconductor nanocrystals (NCs), is reported. The synthesis includes the reaction between tributyl-phosphine selenium and lead 2-ethyl-hexanoate, under inert gas at room temperature. High-resolution transmission electron microscopy (HRTEM), X-ray energy dispersive spectroscopy (EDS), absorption and photo luminescence (PL) spectroscopy were used to characterize the samples. The EDS and HRTEM confirmed the existence of rock-salt cubic structures of the PbSe. Furthermore, the HRTEM showed the formation of PbSe/PbS core-shell structures, with epitaxial shell layer with thickness varying between 1 and 4 ML. The absorption spectra of these materials exhibited distinct transitions, related to the el-h1, e2-h2 and e2-h3 and e2-h1 transitions. These bands are blue shifted with decrease in the NCs diameter. However, the el-h1 is slightly red shifted with increase in the PbS shell thickness. The last effect can be due to an electronic mixing of the PbSe and PbS conduction states. The PL measurements showed a substantial increase of the exciton emission in the core-shell structures, arising by an efficient chemical passivation of the PbSe core. (C) 2002 Elsevier Science B.V. All rights reserved.