2-NO2-5,10,15,20-tetraphenylporphyrin (H2TPP-NO2) is shown to exist in solution as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluorescence spectra of the tautomers contain two well-resolved narrow bands that are slightly (300 cm -1 ) Stokes-shifted with respect to the corresponding QX00 absorption bands, with the spectrum of the more stable tautomer being 500 cm -1 red-shifted as compared to the spectrum of the less stable tautomer. At room temperature, the tautomers show almost identical broad and structureless fluorescence spectra, markedly red-shifted relative to the longest wavelength absorption band. The Stokes shift increases with an increase of solvent polarity, being as large as 2000 cm -1 in N,N-dimethylformamide. Fluorescence lifetimes of the tautomers are found to be markedly (about a factor of 2) different, with both decreasing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical quantum chemical calculations. Charge-transfer (CT) states are found to be located between the porphyrinic Q and B states for both tautomers, and a different energy of the CT states gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the Q X state