Pulse radiolysis of 1:1 complexes of europium(III)-aminopolycarboxylates was studied in aqueous perchlorate solutions to understand relations of the reduction rate of Eu(III) with hydration state and stability of the complex. The rate constant for the reaction of a hydrated electron (e(aq)(-)) with the complex was determined in the temperature range of 278.15 to 333.15 K and the ionic strength range of 0.05 to 1.0 mol kg(-1). The rate constant was linearly related not only with the residual hydration number of the complex representing the hydration state but also with the stability constant and redox potential as equilibrium constants. The reaction of e(aq)(-) with the complex was described by diffusive encounter and successive electron-transfer processes between e(aq)(-) and the complex. Gibbs free energy, enthalpy, and entropy of activation for the reaction were obtained from the effect of temperature on the rate constant, and the entropy term was found to be a dominant factor for the electron-transfer process after the diffusion process. These indicated that the entropy term, reflecting ionic interaction of Eu(III)-carboxylate groups and dehydration of aquo Eu(III) ion on the complexation, caused the difference in the rate constant between the complexes, and linear correlation between the rate constant and hydration number. The effect of ionic strength on the rate constant was examined by using the extended Debye-Huckel equation based on the specific ion interaction theory. Variation of the rate constant with the ionic strength was explained qualitatively by the diffusive encounter, characterized by the product of the expected charges, between e(aq)(-) and the complex. Ion interaction coefficients of e(aq)(-) Eu(III), and activated complexes for the reaction were also discussed.