Abstract. The number of water molecules in the inner-sphere (NH2O) was determined for Eu(III) and the strength of ligand field (RE/M) was evaluated for a variety of coordination environments from the luminescence lifetime and the relative intensity at 615nm and at 592nm, by time-resolved laser-induced fluorescence spectroscopy. When RE/M and NH2O for Eu(III) with a known coordination environment were plotted clear regularity was apparent between the location of the RE/MNH2O plot and the coordination environment of Eu(III). Here, NH2O was calculated by use of the equation, NH2O=9NH2O. Unknown coordination environments of Eu(III) can, in turn, be characterized, including both the inner- and the outer-sphere, simply by plotting RE/M and NH2O for Eu(III) on the diagram. This empirical method is effective for prediction of the coordination environment of hydrated and complexed Eu(III) in solutions and that of the adsorbed Eu(III) on ion-exchange resins and by microorganisms.