The spectroscopic behavior of lumichrome (7,8-dimethyl-alloxazine, LC) in aqueous solutions in a pH range from −1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields, and lifetimes are determined. The ionization stage of ground-state LC changes with rising pH from the cationic form (LCH2+) to the neutral form (LCH) with a mid-point pH of pKc ≈ −0.53, and to the anionic form (LC−) with a mid-point pH of pKa ≈ 12.5. Above pH 7 a partial ground-state tautomerization of LCH to 7,8-dimethyl-isoalloxazine (IAH) occurs by N1–N10 intra-molecular proton transfer. For pH > pKa ≈ 12.5 LCH and IAH change to the anionic forms LC− and IA−, and above pH 14 LC− tautomerizes completely to IA−. In the excited state some neutral lumichrome (LCH*) converts to cationic lumichrome (LCH2+) at low pH by proton transfer from H3O+ to LCH*. No photoinduced excited-state tautomerization of lumichrome was observed. LCH for pH > 3 and IAH are reasonably fluorescent. The fluorescence efficiencies of LC− and IA− are lower than those of LCH and IAH. The fluorescence of LCH2+ is strongly quenched likely by intra-molecular diabatic charge transfer and excited-state relaxation by potential surface touching with the ground state.