The corrected fluorescence maxima of the proton-transfer (PT) tautomer of a novel protein-binding site probe, 4-hydroxy-5-azaphenanthrene (HAP), correlate with the static polarity of the environment. The PT fluorescence of HAP exhibits a maximum in water at 586 nm and at 623 nm in cyclohexane. This PT fluorescence is perturbed by either a strong base or acid, and the emission of the anion or cation shows up in the spectra, respectively. The fluorescence lifetime of the PT tautomer of HAP increases with solvent polarity, being 0.36 ns in cyclohexane and 0.50 ns in methanol. The steady-state and time-resolved data demonstrate the insensitivity of PT tautomer fluorescence of HAP to dipole-dipole relaxation of the environment and the high static polarity of the so-called hydrophobic binding site of human serum albumin (HSA). The overlap of the fluorescence spectra of HAP observed in water solution and in a complex with HSA suggests the similarity of the static polarities in these two environments.