Water soluble CdS quantum dots (QDs) were synthesized by a simple aqueous chemical route using mercaptopropionic acid (MPA) as a stabilizer. These QDs had a fluorescence emission band maximum at 540 nm with a FWHM ∼130 nm and a quantum yield of ∼12%. Transmission electronic microscopy images were used to determine the QD diameter of 8.9 ± 0.4 nm. From this value we calculated the molecular mass M(QD) = 1.17 × 106 g mol-1 and the extinction coefficient at the band edge (450 nm) ε450 = 4.7 × 106 cm-1 M-1, which allowed to determine the true molar concentration of 17 nM for spectroscopic measurements in solution. The fluorescence intensity of MPA-CdS QDs was quenched only in the presence of Co2+ ions, but not in the presence of thirteen other metal cations. The fluorescence quenching of MPA-CdS QDs appeared proportional to the Co2+ concentration in the range 0.04-2 μM. Based on a fluorescence peak position and a lifetime both independent from Co2+ concentration, the quenching mechanism of MPA-CdS QDs appeared static. Because the strong electronic absorption of Co2+ overlaps the emission of QDs, our results can be explained by Förster energy transfer from QD to the bound Co2+ cations.