The excited-state intramolecular proton transfer (ESIPT) of a,series of water-soluble 2-(2'-hydroxyphenyl)benzazole derivatives has been studied under physiological conditions using absorbance and steady-state emission spectroscopy. At neutral pH in the presence of 0.1 M ionic background, the fluorescence properties of these derivatives differ substantially compared to previously reported data in nonaqueous solvents. The ESIPT process is disrupted, presumably due to intermolecular hydrogen bonding with surrounding water molecules combined with increased stabilization of the trans-rotamer, which cannot undergo the ESIPT process. The emission spectrum of the benzimidazole derivative depends significantly on the solvent polarity, as revealed by titrations with Zn(II) in methanol, ethanol, and under physiological conditions, Inhibition of ESIPT via metal coordination shows a significant wavelength shift together with a substantial ratio increase by a factor of 13.7. Titration of the benzoxazole derivative with Zn(II) yielded a 50-fold increased emission intensity. The fluorescence increase is specific for Zn(II), and with a logK of 3.93 (K-d = 117 muM) the ligand would be suitable as a fluorescence probe in a biological environment to gauge Zn(II) concentrations in the range from 10 9muM to 1 mM.