Cadmium sulfide nanoparticles have been synthesized using mercaptoethanol as a capping agent. A refined analysis of the powder X-ray diffraction by Debye function analysis shows that the as synthesized material consists of both similar to 50 wt % hexagonal wurtzite-type nanoparticles of about 2 nn and larger cubic sphalerite-type particles of about 2.6 nm, respectively. Both species have also been identified by high resolution electron microscopy, some of them show multiply twinning with Ei-fold decahedral symmetry. The thermal stability of these nanoparticles is surprisingly high under He. The total mass-averaged size. increases from 2.3 nn (as synthesized) to 3.2 nm after exposure to 315 degrees C in helium for 12 min. However, both polymorphs are still present. Heavy sintering occurs only above similar to 380 degrees C, leading to wurtzite particles of 41 nn on average. These particles are strongly faulted by hexagonal (001) stacking faults (faulting probability = 1.1%)but show negligible twin defects. After synthesis, the particles show the optical absorption peaks at similar to 400 nm. The related optical band gap of 3.1 eV is in excellent agreement with the theoretical tight-binding approximation of Lippens and Lannoo for CdS particles of this size.