The structural environment of silicon dissolved in rutile and α-PbO 2 -structured TiO 2 (TiO 2 -II) was probed using 29 Si MAS NMR on 29 Si-enriched samples. At 1 atm, about 0.01 wt% SiO 2 is incorporated into TiO 2 as IV Si, presumably in interstitial sites. Rutile recovered from 6 GPa, 1600 °C contains about 0.6 wt% SiO 2 , incorporated both as VI Si (~90%) and IV Si (~10%). TiO 2 -II, synthesized at 12 GPa, 1200 °C, contains only VI Si. The chemical shift for VI Si in TiO 2 -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 TiO 2 -SiO 2 solid solution will have to take into account the mixed coordination environment of the Si in TiO 2 , at low pressures.