The synthesis of CdS nanocrystals has been carried out by the thermolysis of cadmium dichloride with trioctylphosphine sulfide as the sulfur source, in tri-n-octylphosphine oxide at 250 °C. The material shows quantum size effects in the optical spectrum. The onset of absorption for CdS is at ≈480 nm (bulk CdS, 515 nm), a blue shift in the absorption edge. The spectrum also shows a second excitonic peak at 379 nm (S. Modes and P. Lianos, J. Phys. Chem., 1989, 93, 5854). The sample is composed of particles of diameter ca. 4.5 nm.