The fluorescence properties of 3-hydroxyflavone isolated in solid Ar at 15 K were investigated. Upon electronic excitation the mols. undergo rapid intramol. H+ transfer. No fluorescence from the excited state of the normal form of the mol. was detected. Perturbations due to H-bonding impurities which produce serious exptl. problems in hydrocarbon glasses are largely suppressed in Ar matrixes. The rise of the green fluorescence of the tautomer was studied with excitation pulses of 230-fs duration and streak camera detection. An apparent tautomer fluorescence rise time of 2.7 ps was obtained by deconvolution. A comparative measurement of the dye coumarin 6 yielded an apparent fluorescence risetime of 2.5 ps, which can be entirely attributed to the group velocity dispersion of the streak camera optics. The rate const. for excited-state intramol. H+ transfer in 3-hydroxyflavone is >1012 s-1.