Amorphous CdSe nanoparticles were prepared by a room-temperature reaction between selenourea and cadmium nitrate and subsequently crystallized by heating at 145 °C in pyridine. Prolonged heating at this temperature did not result in nanocrystal growth, while the addition, before the heating step, of very small amounts of SeO2 dissolved in tetrahydrofuran triggered the dissolution of the small initial nanocrystals and the formation of larger particles. The process is controlled by the relative concentrations of SeO2 and of the capping agent (decanethiol) used in the synthesis of the starting particles. The occurring of an Ostwald ripening mechanism during the treatment with SeO2 was demonstrated by the evolution of the optical absorption spectra, where the sharp excitonic peaks of the initial small nanocrystals progressively disappeared giving rise to broad peaks at longer wavelengths. The results are …