In this work, we use density functional theory to predict the optical properties of TixSi1−xO2 solid solutions. The special quasirandom structure method and the simulated annealing procedure were applied to produce models of crystalline and amorphous TixSi1−xO2. These were fully structurally optimized by using the vasp package, while their electronic structure and optical properties were subsequently calculated by using the wien2k package employing the TB-mBJ potential. The calculated band gaps for a-TixSi1−xO2 evaluated by using the Tauc-like fitting approach are 8.53 eV for SiO2, quickly decreasing to 4.0 eV at x=0.19, 3.52 eV at x=0.34, and 3.24 eV for TiO2. Experimental samples were prepared by means of plasma-enhanced chemical vapor deposition to support the calculations. Ellipsometry and spectrophotometry yield a compositional trend for the experimental optical band gap comparable with our predictions: a quick decrease from 7.94 eV for pure SiO2 to 3.91 eV at x=0.15, followed by a much slower decrease over the rest of the composition range ending at 3.26 eV for pure TiO2. A detailed analysis of anatase and rutile-based solid solutions reveals the introduction of silicon-induced oxygen states into the band gap in the TiO2-rich composition region, which results in the predicted reduction of the band gap. However, we show that the optical absorption of those states is negligible. We have obtained good agreement between the calculated and measured imaginary part of the dielectric function ɛi, especially for the TiO2-rich compositions. Finally, we predict an almost-linear refractive index change at 632.8 nm between a-SiO2 (1.36) and a-TiO2 (2.34), which was experimentally confirmed.