Thermodynamic studies on solvent extraction of Th(IV) have been carried out using TBP and HDEHP as extractants in toluene from aqueous bromide and iodide media containing Arsenazo-III as a colouring reagent. This reagent was used for the spectrophotometric determination of Th(IV). The study was carried out at T = {(283, 298, 313) +/- 1) K to evaluate IgK(ex) as well as the thermodynamic functions Delta G degrees, Delta H degrees, and Delta S degrees of the extraction reactions. The extracted species and the overall extraction mechanisms are postulated. (C) 2008 Elsevier Ltd. All rights reserved.