Fluorescent Pseudomonas species secrete pyoverdin-type siderophores with a high potential to dissolve, bind, and thus transport uranium in the environment. The formation of complexes of UO22 + with pyoverdins released by the groundwater bacterium Pseudomonas fluorescens (CCUG 32456) isolated at a depth of 70 m in the Aspo Hard Rock Laboratory, Sweden, was studied. Mass spectrometry indicated that the cells produce a pyoverdin-mixture with four main components: pyoverdin with a succinamide side chain, pyoverdin with a succinic acid side chain, ferribactin with a succinamide side chain, and ferribactin with a glutamic acid side chain. Three pK values could be determined from the pH-dependent changes in the absorption spectra of the pyoverdin mixture: log beta(012) = 22.67 +/- 0.15 (pK(1) = 4.40), log beta(013) = 29.15 +/- 0.05 (pK(2) = 6.48), and log beta(014) = 33.55 +/- 0.05 (pK(3) = 10.47). The fluorescence properties of the pyoverdin mixture were pH-dependent. The emission maximum changed from 448 nm at pH = 2.1 to 466 nm in the pH 3.8-8.9 range. At pH > 4 a mono-exponential fluorescence decay dominates with a decay time of 5865 +/- 640 ps. A drastic change in the intrinsic fluorescence properties, e.g., static fluorescence quenching, occurred due to the complex formation with UO22+. Species containing UO22+ of the type MpLqHr were identified from the dependencies observed in the ultraviolet visible and time-resolved laser-induced fluorescence spectroscopy spectra at pyoverdin concentrations below 0.1 mM. The following average formation constants were determined: log beta(112) = 30.00 +/- 0.64 and log beta(111) = 26.00 +/- 0.85 at ionic strength I = 0.1 M (NaClO4). The determined stability constants can be used directly in safety calculations of the mobilizing effect of released pyoverdins on uranium, in uranium-contaminated environments such as mine waste disposal sites.