The loading amount and dispersion of tetrahedral species of niobium oxide in the NbOx/SiO2 catalysts could be controlled by adjusting the niobium and ammonia concentrations of impregnating oxalic acid solutions in the preparation by the equilibrium adsorption method. The structures in the solution and the adsorption state of the niobium oxalate complexes were the most important to control the structures of niobium oxide species on the silica surface. The addition of ammonia to the preparation solution affected not only the pH of the solution but also the states of the liquid-solid interface. The former influences the structure and adsorption equilibrium of niobium oxalate complex, while the latter changes the state of adsorption.