Electrochemical studies of the redox mechanisms of niobium chloride and oxychloride in CsCl-NaCl eutectic melts at 550 degrees C are reported. Cyclic voltammograms and electrochemical impedance spectra have been measured in situ over a wide potential range between the limits of chlorine evolution and metal deposition at various oxide concentrations 0 less than or equal to Ox/Nb less than or equal to 3. In analyzing the impedance spectra we have focused on the determination of the Warburg impedance and coefficients, respectively, which reflects the variation of the concentration of redox species as a function of electrode potentials, Taking into account the relevant redox equilibria these Warburg coefficient versus potential curves can be quantitatively described by simple model calculations. This is presented for the first time in detail for a high temperature impedance investigation. In comparison with the corresponding voltammograms this analysis yields the following main results for the redox mechanisms. For pure NbCl5 in CsCl-NaCl eutectic melts evidence is found for a three step reduction mechanism according to Nb(V)-Nb(IV)-Nb(III)-Nb. With oxide addition to a Nb(IV) containing melt oxocomplexes form which are reduced in two steps from Nb(V) to Nb(IV) to Nb-metal or metal oxide deposits. Clear indications for dioxocomplexes are derived from the impedance analysis which form in melts with Ox/Nb = 2. Comparison with the corresponding melts containing tantalum chloride or oxychloride is discussed where dioxocomplexes are not found.