Ubiquitous fluorescent Pseudomonas species secrete bacterial pyoverdin-type siderophores. These bioligands have great potential to bind and transport actinides in the environment due to their hydroxamate and catechol functionalities. We investigated the unknown interaction of the neptunyl cation (NpO2+) with pyoverdins (PYO) released by Pseudomonas fluorescens (CCUG 32456) cells and with simple hydroxamate (salicylhydroxamic acid: SHA and benzohydroxamic acid: BHA) and catechol (2,3-dihydroxynaphthalene: NAP) ligands using near-infrared (NIR) absorption spectroscopy over a wide pH range. NpO2+-bioligand species of the MxLyHz type were identified from the spectrophotometric titrations in all four systems. The 1 : 1 : 2, 1 : 1 : 1, and 1 : 1 : 0 complexes were determined with the pyoverdins. In addition to 1: 1 species, SHA, BHA, and NAP also form 1: 2 : 0 species with NpO2+. The stability constants of these neptunyl(V)-bioligand complexes and their individual spectroscopic properties are reported. Our findings indicate that NpO2+ has a stronger affinity to the catechol functionality of the pyoverdin molecule. The identified NpO2+-PYO species belong to the strongest NpO2+ complexes with organic material reported so far.