We investigated the chemistry of singlet oxygen with a cadmium-sulfur cluster, (Me4N)(2)[Cd-4(SPh)(10)]. This cluster was used as a model for cadmium-sulfur nanoparticles. Such nanoparticles are often used in conjunction with photosensitizers (for singlet oxygen generation or dye-sensitized solar cells), and hence, it is important to determine if cadmium-sulfur moieties physically quench and/or chemically react with singlet oxygen. We found that (Me4N)(2)[Cd-4(SPh)(10)] is indeed a very strong quencher of singlet oxygen with total rate constants for O-1(2) removal of (5.8 +/- 1.3) x 10(8) M-1 in acetonitrile and (1.2 +/- 0.5) X 10(8) M-1 s(-1) in CD3OD. Physical quenching predominates, but chemical reaction leading to decomposition of the cluster and formation of sulfinate is also significant, with a rate constant of (4.1 +/- 0.6) x 10(6) s(-1) in methanol. Commercially available cadmium-sulfur quantum dots ("lumidots") show similar singlet oxygen quenching rate constants, based on the molar concentration of the quantum dots.