High-field 17O MAS NMR was used to investigate the binding of self-assembled monolayers of 17O-enriched phosphonic acids deposited on a titania anatase support. The spectra were recorded at two different magnetic fields (9.4 and 17.6 T), to improve the reliability of the simulations of the different resonances. The spectra recorded at 17.6 T offer an excellent resolution between the different oxygen sites, PO, P−O−H, and P−O−Ti, thus greatly facilitating their quantification. The data reported here give direct evidence of the extensive formation of Ti−O−P bonds in the surface modification of titania by phosphonic acids. The presence of residual PO and P−O−H sites indicates the presence of several different binding modes in phosphonic acid monolayers. The chemical shift of P−O−Ti sites is consistent with bridging (as opposed to chelating) modes.