The equilibrium constant for the chloro complex formation of Nb(V) NbCl6-<-> NbCl5+Cl- (i) in NaCl-AlCl3 melts at 175 degrees C was found to be pK(i) = 2.86(5). The oxochloro complex formation of Nb(V) and Ta(V) in NaCl-AlCl3 melts at 175 degrees C could be explained by the following equilibria: MOCl4-<-> MOCl3+Cl- (ii) MOCl3<-> MOCl2++Cl- (iii) where M = Nb and Ta. The equilibrium constants determined by potentiometric measurements with chlorine-chloride electrodes were, for M = Nb, pK(ii) = 2.21(4) and pK(iii) = 3.95(5) and, for M = Ta, pK(ii) = 2.743(15) and pK(iii) = 4.521(13). NbCl6- has two bands in the UV-vis region, a strong one at 34.7 x 10(3) cm(-1) and a weaker one at 41.6 x 10(3) cm(-1). The MOCl4- complexes showed in the case of Nb(V) absorption bands at 32.7 and 42.9 x 10(3) cm(-1) and in the case of Ta(V) at 38.6 and 48.1 X 10(3) cm(-1).