Abstract— <p>Thermolysis stages of Nb forms of polyantimonic acid that crystallize in the pyrochlore structure and composites based on polyantimonic acid containing Nb⁵⁺ ions have been identified in the temperature range 24–700°C. In the INTRODUCTION section, we depict distinctive structural features of polyantimonic acid: the presence of water molecules on particular crystallographic sites and adsorbed water molecules. In connection with this, the objectives of this work were to determine the amount of water of hydration in H₂Sb_{2–<i>х</i>}Nb_<i>x</i>O₆∙<i>n</i>H₂O samples and model the thermolysis process. Using a number of experimental techniques—X-ray diffraction, thermogravimetry, and IR spectroscopy—we have identified thermolysis stages, determined the phase composition of the thermolysis products, and proposed structural formulas of the synthesized compounds with allowance for the thermolysis model. The amount of water of hydration in the sample with the highest dopant concentration, <i>x</i> = 0.4, has been shown to be <i>n</i> = 5.34; the undoped sample, with <i>x</i> = 0, has <i>n</i> = 3.10. The content of water of hydration in the <i>x</i> = 0.6 composite, having the best proton-conducting properties, is <i>n</i> = 4.85. It seems likely that the proton conduction mechanism is influenced by the nonequivalence of the proton-containing groups, as evidenced by thermal analysis and IR spectroscopy data.</p>