Abstract An FTIR method has been developed to simultaneously evaluate the refractive index and thickness of the oxide film on ZrNb alloys. Interference data was collected at several angles of incidence, using s-polarization. From this interference data, the refractive index was calculated for a range of thickness estimates. The degree of scatter between this refractive index data and a dispersion curve fit to the data set was minimized to determine both the oxide thickness and refractive index. The method was applied to the oxide layers grown on Zr2.5 Nb annealed ( α-Zr + β- Zr), Zr20Nb annealed (β-Zr) and Zr20Nb annealed and aged ( α- Zr + β- Nb). The refractive index of the oxide grown on β-Zr (≈2.73) was significantly different than that of the oxide grown on α-Zr (≈2.08). The oxide thicknesses determined by this method were in good agreement with weight gain and metallography results.