In this paper, a method for determination of tantalum in ores by direct potentiometry using an ion selective membrane electrode sensitive to the ion TaF6̅ has been proposed. After checking the Nernstian behavior of the electrode in the presence of TaF6̅ , the influence of NbF6̅ concentration on the determination of tantalum was also investigated. Potentiometric measurements on synthetic solutions of TaF6̅ 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 NbF6̅ as interferering ions. From the results recorded, it appeared that the response of the electrode deviate to the Nernstian behavior when TaF6̅ concentration is less or equal to 5×10-5 mol.l-1 corresponding to a concentration of NbF6̅ at least twenty times greater than that of TaF6̅. Using the standard addition method, it was observed that the ion NbF6̅ is no longer interferering with TaF6̅ on the entire range of concentration of TaF6̅ 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.