Nanocrystalline ZnS and CdS samples have been synthesized in the presence of Eu3+ and Tb3+ using various techniques (precipitation in water, methanol, or toluene and inverse micelle techniques) which have been reported to yield ZnS or CdS nanoparticles doped with luminescent rare earth ions. Nanocrystalline particles with a typical diameter of 4 nm were formed. In some cases, particles were heated (up to 800 °C) which resulted in an increase of the particle diameter (> 20 nm). To study the incorporation of rare earth ions in the particles, luminescence spectra have been measured. Upon excitation in the semiconductor host lattice, no emission or weak emission is observed from the rare earth ions. The excitation spectra of the characteristic rare earth emissions show excitation lines corresponding to intraconfigurational 4fn-4fn transitions of the rare earth ions but not the semiconductor host lattice excitation band. The absence of a host lattice excitation band indicates that with the presently used synthesis techniques the rare earth ions are not incorporated in the nanocrystalline semiconductor particles but are probably adsorbed at the surface.