We report the fabrication and characterization of benzoic acid ion exchanged planar waveguides in lithium tantalate in order to investigate the conditions suitable for achieving quasiphase matching. Samples from az‐cut lithium tantalate wafer were ion exchanged at 205 °C for 3 or 6 h and annealed for between 16 and 120 s at temperatures of between 465 and 594 °C. The anneal diffusion coefficient at a temperature of 525 °C was measured to be 264 &mgr;m2/h. Domain inversion was observed for anneals at 594 °C/30 s and at 594 °C/60 s. ©1995 American Institute of Physics.