The presence of the two isomeric species (M and m) of Eu(III) (DOTA-like) complexes has been detected in aqueous solution by UV-Vis absorbance spectrophotometry. It shows that the 7 F 0 -> 5 D 0 transition in Eu(III) complexes does not only depend on the coordinating atoms present in the first coordination sphere but that it may also be influenced by small differences in the coordination geometry. The absorbance spectra could be well fitted with the K=M/m equilibrium constants measured by NMR. The wavelength variation between the two isomers is found to be 0.20 nm (DTMA), 0.14 nm (DOTA) and about 0.25 nm (DOTAM) (DTMA=1,4,7,10-tetrakis-(methylcarbamoylmethyl)-1,4,7,10-tetraazacyclo-dodec ane; DOTA=1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)-cyclododecane; DOTAM=1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane). This is small enough not to affect the quantitative determination of water coordination equilibria in solution as hydration equilibria produce differences of about 0.5 nm.