The excited state intramolecular proton transfer reaction of 4'-N,N-diethylamino-3-hydroxyflavone in imidazolium-based room temperature ionic liquids was investigated by time resolved fluorescent measurements. Dual emission from the normal and the tautomer forms were observed. The normal emission peak position gradually shifts to longer wavelength in the sub ns time region, and the tautomer emission was delayed relative to the normal emission. These results suggest that solvation processes in the earlier time region after excitation play an important role in the proton transfer mechanism. (C) 2008 Elsevier B.V. All rights reserved.