The solvent and pH dependence of the absorption and fluorescence spectra, fluorescence quantum yields and lifetimes, and fluorescence quenching behaviour of 4-(2,3,5-tri-O-acetyl-β-D-ribofuranosylamino)pyrido[2,1-h]pteridin-11-ium-6-olate, its α-anomer, the parent ribonucleoside 4-(β-D-ribofuranosylamino)-pyrido[2,1-h]-pteridin-11-ium-5-olate (luminarosine) and the aglycone 4-aminopyrido[2,1-h]-pteridin-11-ium-5-olate (luminarine) are reported. Spectroscopic properties of these compounds in solvents of different polarities are characterized by evidence of the occurrence of a highly dipolar, charge-transfer excited state within the heterocyclic betaine system. The pH dependence of the absorption spectra of luminarosine and luminarine infer the occurrence of ground state prototropic equilibria in solution involving protonation of the negatively charged oxygen atom followed by tautomerism to a lactam. The excited state prototropic equilibria, as shown by fluorescence properties, seem to be more complex and suggest the possible occurrence of proton-transfer and/or phototautomerization reactions.