The optical constants of Co3O4 films were determined by analyzing variable angle of incidence spectroscopic ellipsometry data and normal incidence transmittance data, between 3500 and 17 000 A. Absorption was taken to be nonzero for the soda–lime–silica float glass substrate. The absorption of the soda–lime–silica float glass was fitted by an ensemble of four Gaussian oscillators over the entire measured spectral range. Surface roughness of the film was measured with atomic force microscopy and included in the optical model. The films were deposited by spray pyrolysis of cobalt acetylacetonate onto heated soda–lime–silica float glass and fused silica substrates. Fused silica was used to observe film absorption without the effect of substrate absorption. The oxide film phase was identified with thin‐film x‐ray diffraction analysis. Simultaneous fits of ellipsometry and transmittance data from coated samples were computed to obtain the film thickness, optical constants, and the amplitudes of the four Gaussi...