Optical, mechanical, and thermal properties of optical thin films are very important for a reliable device performance. In the present work, the effect of annealing on the stability and the characteristics of niobium and tantalum oxide films grown at room temperature (RT) by dual ion beam sputtering were studied. The refractive index ($n(\lambda )$), extinction coefficient ($k(\lambda )$), hardness ($H$), reduced Young’s modulus ($E_r$), and film stress ($\sigma$) were investigated as a function of the annealing temperature ($T_A$). X-ray diffraction analysis showed that all as-deposited films were amorphous, and crystallization was observed only after annealing at 700°C. Compositional analyses confirmed that the atomic ratio of oxygen to metal in as-deposited and annealed films was close to 2.5, indicating that the films were stoichiometric pentoxides of Nb and Ta. The properties of ${\mathrm{Nb}}_2{\mathrm{O}}_5$ and ${\mathrm{Ta}}_2{\mathrm{O}}_5$ films were, respectively, affected by postdeposition annealing: $n(\lambda )$ values (at 550 nm) decreased from 2.30 to 2.20 and from 2.14 to 2.08, the average $H$ and $E_r$ values increased from 5.6 to 7.4 GPa, and from 121 to 132 GPa for ${\mathrm{Nb}}_2{\mathrm{O}}_5$, and from 6.5 to 8.3 GPa, and from 132 to 144 GPa for ${\mathrm{Ta}}_2{\mathrm{O}}_5$, and the initial low compressive stress for both materials changed to tensile. We explain the variation of the coating material properties in terms of film stoichiometry, crystallinity, electronic structure, and possible reactions at the film–substrate interface.