The reaction of chlorine with columbite concentrate, a niobium and tantalum ore, was studied by thermogravimetry between 300 degrees and 950 degrees C. Nonisothermal and isothermal measurements were performed. Morphological evolution of solid reactants and elemental composition of particles were analyzed by SEM and EDXS, respectively. The growth of crystals, high in Ta and Nb content, was observed. The Ta content in remaining samples was greater as the chlorination temperature increased. A scheme of the reaction mechanism is proposed.