Steady-state and time-resolved UV - vis spectroscopies were used to study the photoprotolytic properties of lumazine, which belongs to a class of biologically important compounds - the petridines. We found that in water an excited-state proton transfer occurs with a time constant of ∼70 ps and competes with a nonradiative rate of about the same value. The nonradiative rate of the protonated form of lumazine in polar and non-polar solvents is large knr ≥ 1.5 × 1010s-1. The fluorescence properties indicate that in water, the ground-state neutral form of lumazine is already stable in two tautomeric forms. The fluorescence of the deprotonated form is quenched by protons in acidic solutions with a diffusion-controlled reaction rate. We conclude that the neutral form of lumazine is an irreversible mild photoacid.