Diffusion coefficients of Nb(V) and Nb(IV) fluoride complexes in chloride and fluoride melts were obtained by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. The values found by these methods are in good agreement with each other. Decreasing of diffusion coefficients was obtained when the cation in the second coordination sphere changes from Na+ to Cs+. It was determined that diffusion coefficients decrease when chloride melts are converted to fluoride melts. Optimized geometrical structures and energies of such model particles as (M+)(n)[Nb(V)F-7](2-) and (M+)(n)[Nb(IV)F-7](3-) (M+ = Na+, K+, Cs+; n = 1-6) were obtained by quantum-chemical calculations. There most stable compositions of these particles have been determined. Energies of both the [NbF7] complexes for all outersphere compositions, of the [NbF7] complex bond with its outersphere shell, and the (Nb-F) and (Nb-M) bond lengths have been calculated. Variations of specified quantities in the Na+-K+-Cs+ and Nb(IV)-Nb(V) series have been compared with changes in diffusion coefficients. This has allowed to make some guesses concerning the mechanism of the [NbF7] complex diffusion in melts. (C) 2014 The Electrochemical Society. All rights reserved.