The photochemical reactions of the 1-deaza derivative of riboflavin (RF) have been studied by sub-ps time-resolved fluorescence spectroscopy. A potential effect of the solvent polarity and hydrogen bonding capability on the excited state (ES) dynamics was tested by measuring 1-deaza RF in water and, after acetylation of the sugar moiety, also in acetonitrile. Opposite to former reports, which indicated no fluorescence after laser excitation (Spencer et al. [1977] Biochemistry16: 3586-3594), we find a significant fluorescence in either solvent. For both compounds, the fluorescence decays are biexponential with lifetimes in the ps time domain (1.8 and 12.5 ps for 1-deaza RF, and 3 and 87 ps for Ac4-1-deaza RF). In addition, a third, independent fluorescence decay was observed for both compounds. The remarkably slower second decay process for the measurements of the tetra-acetyl derivative in acetonitrile points to the involvement of hydrogen bonding or changes in the protonation/tautomerization state in the ES. In contrast to the parent compound (RF), no triplet state formation can be detected for 1-deaza RF, making the internal conversion and the fluorescence the only decay processes of the ES.