We present studies of temperature viscosity and polarity effects on the photodynamics of l'-hydroxy-2'-acetonaphthone. We observed a striking temperature effect on the non-radiative decays and which explains the low emission quantum yield of the formed keto-type phototautomers. The thermal activation energy is lower for the keto structure (0.96 kcal/mol) than for the keto rotamer one (3.4 kcal/mol). For the alcohols family of solvents, the keto-type tautomer is more sensitive to solvent response than the keto rotamer. However for non-hydroxylic media the low influence of solvent polarity is similar for both keto type structures. (C) 2003 Elsevier B.V. All rights reserved.