In the emission decay kinetics of Eu(III)-2-thenoyltrifluoroacetone (Htta) aqueous system, we found out a new quenching mechanism due to the tta(-) dissociation from Eu(tta)(2+) in the excited state. The mechanism well explained the double-exponential decay of the fluorescence observed in the Eu(III)-Htta system. The analysis of the decay curve observed at various temperatures gave the rate constants and thermodynamic parameters for the tta(-) dissociation from Eu(tta)(2+).