Aiming to study the solid-state diffusion of lithium into pure and lithium doped niobium pentoxide (Nb2O5) thin sol-gel films were investigated by galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The obtained values of the diffusion coefficient were 2.5 x 10(-11) at x = 0.83 and 7.4 x 10(-13) at x = 1.65 for pure and doped Nb2O5 films respectively. The impedance results of both kinds of films adjusted by equivalent circuits showed very good agreement between experimental and theoretical data.