Metal-specific fluorescence probes are of increasing importance in understanding the neurobiology and general cell biology of zinc. Several quinoline-based compounds such as TSQ and zinquin have been employed to detect zinc in fluorescence microscopy experiments in vivo; however, the aqueous solution chemistry remains equivocal. In some cases, this family of probes is said to reveal labile pools of Zn(II) inside the cell, yet in other cases, these probes ate suggested to remove Zn(II) from tightly bound sites in proteins. Since the binding modes, coordination numbers, and thermodynamics of zinc-zinquin interactions in aqueous solution have not been established, these proposals are difficult to distinguish. Here we show that, under physiological conditions, the various forms of zinquin bind Zn(II) with a high degree of cooperativity forming 2:1 complexes. Potentiometric, UV-visible, and fluorescence methods all yield an overall binding constant of log K = 13.5 under physiological conditions. To put this number in perspective with other Zn chelators and biological ligands, we compare the calculated so-called pM values (-log[Zn(II)](free)) for a series of compounds with different stoichiometries under a typical condition. The pZn value for zinquin (9.3) is similar to that of EGTA. (9.5) but much smaller than the value for carbonic anhydrase (12.4) or EDTA (14.3) and, thus, serves as a useful gauge for comparing zinc affinities. With respect to in vivo applications of zinquin, such as intracellular fluorescence microscopy studies, we End that the typical detection limit for free Zn(II) in aqueous solution is 4 phl, or 0.3 parts per trillion, at pH 7.2. These results have implications for the availability of zinc in various intracellular compartments.