Flow injection electrospray ionization mass spectrometry has been shown to produce simple, characteristic m/z signals for Th-hydroxamate siderophore (desferrioxamine and ferrichrome) complexes, with Th complexed as a simple 4+ ion in the environmentally relevant pH range investigated (pH 5-9). All species of interest for this study were identified optimally in the positive mode; thus, multiple species were analyzed concurrently in a single spectrum. Complexation of Th by the two siderophores was rapid in 1:1 molar aqueous solution, reaching equilibrium before the first measurement was possible at 2 min. However, a significant proportion of the equimolar siderophore remained uncomplexed. Both siderophores rapidly exchanged Th for Fe when equimolar Fe(III) was added to the Th complexes, and only a small proportion of each siderophore remained complexed with Th at equilibrium (7-30 min). The results show a difference in the affinities of the two siderophores for the metals; ferrichrome has a 5-fold higher affinity than desferrioxamine for Th and a 5-fold lower affinity than desferrioxamine for Fe. Also, siderophore-complexed Th interacted strongly with a cation-exchange resin suggesting that, even when complexed by trianionic siderophores, Th mobility will be impeded by interactions with negatively charged binding sites in subsurface environmental matrixes. These results have important implications regarding siderophore-enhanced actinide(IV) mobility in the terrestrial environment.