By means of pH-potentiometric titration, the processes of the complexes formation in the system Nb6O198-WO42-H+-H2O with CNb : CW = 2: 4 was studied at different Nb + W concentrations. Exerimental data, being processed by mathematical modeling, allowed to obtain the distribution diagrams of individual niobium and tungsten isopoly anions, and mixed isopolyniobotungstates in the range of Z = C0H+/C0Nb+W = 0 - 2.0 (background electrolyte is NaCl). Concentrational and thermodynamic formation constants were calculated using quasi-Neuton method (CLINP 2.1 software) and it was shown, that the formation of isopolyniobotungstates (HxNb2W4O19(4-x)-, x = 0 - 2) of the 6th row of Periodic table proceeds through intermediate Nb3W3O195- ion formation. Thallium salts Tl3HNb2W4O19 • 10H2O and Tl2H2Nb2W4O19 • 10H2O were isolated and characterized by elemental and EDX spectral analysis, electron microscopy and FTIR-spectroscopy.