Thin films of zirconium niobium oxide (Zr 0.7 Nb 0.3 O 2 ) were deposited by DC reactive magnetron sputtering method on unheated quartz and p-silicon substrates at different oxygen partial pressures. XPS studies confirmed the presence of zirconium, niobium and oxygen associated with Zr 0.7 Nb 0.3 O 2 by showing the respective core-level binding energy values. The films formed at oxygen partial pressure of 4 × 10 -4 Torr were of single-phase Zr 0.7 Nb 0.3 O 2 with amorphous nature. Optical band gap of the films increased from 4.19 to 4.42 eV with an increase in oxygen partial pressure from 8 × 10 -5 to 4 × 10 -4 Torr. The Zr 0.7 Nb 0.3 O 2 films formed at 4 × 10 -4 Torr were also annealed in air at different temperatures in the range from 500 to 750 °C. The films annealed at temperature 600 °C showed a weak diffraction peak of tetragonal Nb 2 O 5 with amorphous background. Further increase in temperature to 750 °C, the films transformed to polycrystalline with tetragonal structure. Shift in the diffraction angles revealed that niobium substituted the zirconium and form Zr 0.7 Nb 0.3 O 2 . The band gap of the films increased from 4.64 to 4.81 eV with the increase in annealing temperature from 600 to 750 °C. Metal-oxide-semiconductor (MOS) gate capacitors with configuration Al/Zr 0.7 Nb 0.3 O 2 /p-Si were deposited and studied the capacitance-voltage and current-voltage characteristics. The dielectric constant of the films increased from 15 to 23 with increase in annealing temperature from 600 to 750 °C. The leakage current density of the as-deposited MOS capacitors was 2 × 10 -5 A/cm 2 and decreased to 4 × 10 -7 A/cm 2 with the increase in annealing temperature to 750 °C due to improvement in the crystallinity and decrease in defect density in the films.