The structural environment of silicon dissolved in rutile and α-PbO2-structured TiO2 (TiO2-II) was probed using 29Si MAS NMR on 29Si-enriched samples. At 1 atm, about 0.01 wt% SiO2 is incorporated into TiO2 as IVSi, presumably in interstitial sites. Rutile recovered from 6 GPa, 1600 °C contains about 0.6 wt% SiO2, incorporated both as VISi (~90%) and IVSi (~10%). TiO2-II, synthesized at 12 GPa, 1200 °C, contains only VISi. The chemical shift for VISi in TiO2-II is slightly less negative than that for rutile, and the peak is split, suggesting either a more complex mechanism of substitution or a different response to quenching or decompression in the lower-symmetry structure. Future thermodynamic studies of the TiO2-SiO2 solid solution will have to take into account the mixed coordination environment of the Si in TiO2, at low pressures.