In this paper, a method for determination of tantalum in ores by direct potentiometry using an ion selective membrane electrode sensitive to the ion TaF 6 has been proposed. After checking the Nernstian behavior of the electrode in the presence of TaF 6 , the influence of NbF 6 concentration on the determination of tantalum was also investigated. Potentiometric measurements on synthetic solutions of TaF 6 in the concentration range from 10 -3 to 10 -5 mol.l -1 were performed in the presence of 10 -3 mol.l -1 of NbF 6 as interferering ions. From the results recorded, it appeared that the response of the electrode deviate to the Nernstian behavior when TaF 6 concentration is less or equal to 5×10 -5 mol.l -1 corresponding to a concentration of NbF 6 at least twenty times greater than that of TaF 6 . Using the standard addition method, it was observed that the ion NbF 6 is no longer interferering with TaF 6 on the entire range of concentration of TaF 6 investigated. The accuracy of the method conducted on a series of potentiometric measurements was about 1%. The technique developed has then been applied to the determination of tantalum in samples of Colombo-tantalite from various region of Rwanda.