Wadsley–Roth phase Li x Nb9PO25 has been studied as a potential candidate for anode material of Li-ion batteries. Its crystal structure, which consists of ReO3-type blocks of NbO6 octahedra connected with PO4 tetrahedra, provides a good stability and performance during Li+ insertion/removal. Li-ion chemical diffusion coefficient (D chem) in Li x Nb9PO25 was determined by means of potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. Different data treatments (classical Warburg equation or the model of an electrode system with ohmic potential drop and/or slow kinetics of the interfacial Li+ ion transfer across the electrode/electrolyte interface) were used for calculation of D chem of the Li ion inside this material; their applicability is discussed in the article. D chem changes with the Li-ion doping degree, x, in Li x Nb9PO25 and has a sharp minimum near the two-phase region at appr. 1.7V vs. Li+/Li. These values of D chem in Li x Nb9PO25 (∼10−9–10−11 cm2 s−1) were found to be in average noticeably higher than in the widely studied anode material, Li4Ti5O12.