Abstract: The chloride, bromide and iodide complexes of Th4+ were studied in an acidic perchlorate medium of ionic strength μ = 4 at 25℃, using the cation-exchange methods of S. Fronaeus and of J. Schubert.The stability constants obtained were β1 = 1,3, β2 = 0.6 and β3 = 0.1 for the chloride system; β1 = 0.4 and β2 = 0.3 for the bromide system; and β1 = 0.3 for the iodide system. From these results it is indicated that thorium belongs to S. Ahrland's A-type acceptor atoms, since the stability constant of the complex species decreases in the 6rder. F-l~>Br~>I-.