hermochromic Mg-doped VO2 films were deposited by reactive DC magnetron sputtering onto heated glass and carbon substrates. Elemental compositions were inferred from Rutherford backscattering. Optical band gaps were obtained from spectral transmittance and reflectance measurements—from both the film side and the back side of the samples—and ensuing determination of absorption coefficients. The band gap of Mg-doped films was found to increase by 3.6 ± 0.6 eV per unit of atom ratio Mg/(Mg + V) for 0 < Mg/(Mg + V) < 0.21. The presence of ~0.45 at.% Si enhanced the band gap even more.