Optical mode measurements have been used to determine the anisotropic diffusion coefficient and surface index changes for planar Ti : LiNbO3waveguides at 0.633 &mgr;m. Measured values for the diffusion coefficient at 1000 °C areDy=9.4×10−13andDz=1.4×10−12cm2/sec. The ratio of the extraordinary to the ordinary surface index change (&Dgr;ne/&Dgr;n0) was found to vary between 1.3 and 1.8, depending on diffusion temperature and Ti film thickness. Ti and Li concentration profiles were measured by secondary‐ion mass spectrometry. Additional high concentration peaks, 0.2–0.3 &mgr;m wide for both ions, were found superimposed on otherwise well‐behaved diffusion profiles. This observation is interpreted to result from a tendency towards Li‐Ti‐O compound (Li2TiO3or Li2Ti3O7) formation at the diffusion temperature in a dilute mixture with LiNbO3. Such compound formation has the effect of impeding the Ti diffusion into the LiNbO3substrate. Electron microprobe measurements were used to measure lateral diffusion from channel waveguides. Under nominally identical conditions, lateral diffusion coefficients varying by as much as a factor of 3 have been observed. The minimum lateral diffusion coefficients observed at 1000 °C wereDy=9.7×10−13andDz=1.4×10−12cm2/sec.