The fluorescence lifetimes of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) were determined in a large number of solvents and solvent mixtures, displaying variations from 825 ns in CD3CN to 13 ns in CHCl3. Large deuterium isotope effects (8-11) and significant activation energies (generally 2-15 kJ mol(-1)) for fluorescence quenching of DBO are observed, which support a novel solvent-induced quenching mechanism via an aborted hydrogen atom transfer. In CCl4 a photoinduced electron-transfer quenching mechanism may compete. The fluorescence quenching by solvent mixtures is not proportional to the mole fraction of the two components, indicating a preferential solvation of the excited state.