The solution thermodynamics of water-soluble complexes formed between Ce(III), Ce(IV), Th(IV) and the octadentate chelating agent 3,4,3-LI(1,2-HOPO) were investigated. Several techniques including spectrofluorimetric and automated spectrophotometric titrations were used to overcome the slow spontaneous oxidation of Ce(III) complexes yielding to stability constants of log β 110 = 17.4 ± 0.5, log β 11−1 = 8.3 ± 0.4 and log β 111 = 21.2 ± 0.