We have investigated the structure of Eu(III)-citrate complexes in aqueous solution and their degradability by Pseudomonas fluorescens. Analysis of 1 : 1, 1 :2, and 1 :5 Eu(III): citrate solutions at pH 7 by electrospray ionization mass spectrometry (ESI-MS) showed that the 2: 2 Eu(III)-citrate complex is the predominant complex species at a low citrate/Eu(III) ratio, while at a high ratio, a 1 :2 Eu(Ill)-citrate complex is formed preferably. Studies on the biodegradation of Eu(III)-citrate complex by P. fluorescens have shown that a 2: 2 Eu(ill)-citrate complex is resistent to degradation while a 1 :2 complex transforms to a 2:2 complex with the degradation of excess citric acid.