The photophysical properties of tyrosine and its derivatives with free and blocked functional groups in water were studied by steady-state and time-resolved fluorescence spectroscopy and global analysis. Tyrosine fluorescence intensity decays in water at pH = 5.5 in the short-wavelength region (290−320 nm) are monoexponential, whereas, at longer wavelengths, they are biexponential. The monoexponential fluorescence intensity decay of O-methyl tyrosine across the fluorescence band is observed. The fluorescence lifetimes of Tyr calculated using a global analysis are equal to 3.37 ± 0.04 ns at the short-wavelength region and 0.98 ± 0.12 ns at the longer-wavelength region. This observation, together with the decay-associated spectra, indicate that the short-lifetime component can be attributed to tyrosine with phenol hydroxyl groups hydrogen-bonded with water molecules. The rotamer populations calculated from potentials of mean forces, as well as those obtained from 1H NMR spectroscopy, do not correspond to th...