We carried out the transient Raman spectroscopic studies to investigate the relaxation process of the excited-state intramolecular reverse proton transfer (r-ESIPT) in 1-hydroxyanthraqui none. To assign the observed excited-state vibrational modes, we carried out the mode analysis through the ab initio calculation with HF/4-31G(d) and CIS/4-31G(d) methods. The frequency changes of the proton-transfer-related Raman modes provide more quantitative feature on the r-ESIPT process in the excited triplet-state. Especially, the solvent natures affect the relaxation process of the excited keto-tautomer. (C) 2004 Elsevier B.V. All rights reserved.