Surface modification with thiols and redispersion into non-micellar solvents of CdS ultrafine particles prepared in situ in reverse micellar systems have been investigated. An excellent redispersion in pyridine is observed when the CdS ultrafine particles are prepared with 0.4 < y (= [S2-]/[Cd2+]) < 0.9 owing to the binding of the added thiophenol molecules to the excess Cd2+ ions adsorbed to the surface of the CdS particles. The particle redispersion ratio, R, is defined as the ratio of absorbance of ultrafine particles redispersed in nonmicellar media to that of particles in the same amount of the original reverse micellar solution. As the time after CdS formation until the addition of thiophenol to the micellar solution (t(SH)) increases, the redispersion ratio R into either pyridine or CH2Cl2 decreases. The emission at lambda(em)=570 nm of the formed CdS particles in reverse micellar solutions before adding thiophenol, which has been attributed to surface sulfur vacancies of CdS, also decreases rapidly after the particle formation. This observation suggests that the surface sulfur vacancies of CdS play an important role in the surface modification by thiols, In the redispersion process of CdS ultrafine particles, the added thiophenol molecules are considered to have two functions : destabilizing the reverse micelles and binding to the surface of the particles ejected from the micelles owing to destabilization. Solvent molecules with both ligand parts and hydrophobic groups are favorable for redispersing the particles covered with thiophenol.