The application of solvent extraction measurements to the determination of the degree of polymerization of metal chelate compounds was investigated using the 1 : 1.5 Th(IV)pyrocatechol-3,5-disulfonate (Tiron) system. The distribution of Th/sup 234/ tracer between a cyclohexane phase containing thenoyltrifluoroacetone and an aqueous phase containing Tiron was determined at various thorium concentrations. Mathematical treatment of the data obtained has shown that the 1: 1.5 Th(IV)-Tiron chelate is binuclear at pH 4.3.(auth)