[en] A procedure for transforming thorium sulfate into thorium nitrate by means of a strong cationic ion exchanger is presented. The thorium sulfate solution (approximately 15 g/L Th (SO₄) ₂) is percolate through the resin and the column is washed first with water, with a 0,2 M N H₄ OH solution and then with a 0.2 M N H₄ NO₃ solution in order to eliminate sulfate ion. Thorium is eluted with a 2 M solution of (N H₄) ₂ CO₃. This eluate is treated with a solution of nitric acid in order to obtain the complete transformation into Th (NO₃) ₄. The proposed procedure leads to good quality thorium nitrate with high uranium decontamination. (author)