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This letter deals with the experimental observation of oscillations in the infrared reflectance from Nb ultra-thin films deposited on α-type SiO2 substrates. P-polarized reflectance (Rp) measurements are made using a tunable p-polarized CO2 waveguide laser using wavelengths between 9.2 and 10.4 μm. Several Nb/SiO2 quantum wells were specially made by the RF sputtering technique. Tailored thicknesses run between 5.5 and 55 Å. Because of the strong influence from the chosen substrate, IR reflectivity was fitted to the optical response of our metal-substrate system by using the three-oscillator model and numerical calculations on the basis of the local field calculation for a single metallic quantum well. Although quantum size effects are well studied in semiconductor compounds, there are only a few studies of this effect in metallic films where the present investigation has its most important contribution. [ABSTRACT FROM AUTHOR]