A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum α-(AlxGa1−x)2O3 thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular (ε∞,⊥) and parallel (ε∞,∥) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the high-frequency dielectric constants with b⊥=0.386 and b∥=0.307.