Received December 28, 1981 Accepted for Publication March 22, 1982 Two 5-m pulse columns with the same cartridge geometries have been installed for Thorex process studies. First, the differences of the aqueous and the organic continuous modes of operation have been investigated. Flow characteristics and extraction efficiencies of acetic acid and thorium nitrate have been obtained. Then, the uranium and thorium coextraction and partitioning have been carried out. The concentration profiles of thorium, uranium, and nitric acid along columns have been determined for the coextraction and the partitioning process and the Height Equivalent Theoretical Stage values have been calculated from these data.