As stoichiometric LiTaO3 (LT) draws a considerable attention for integrated optical waveguide devices, we have investigated Zn diffusion into this material by diffusing 70-nm-thick ZnO films deposited on y-cut LT substrates at 700-900 degreesC under various atmospheres. It was observed that the surface quality was very sensitive to pressure, but weakly affected by other diffusion conditions such as temperature and atmosphere. While the surface degraded, being covered with some residuals after heat treatment at the atmospheric pressure, it was very smooth and clear when the pressure was lowered below about 10 Torr. Another feature of Zn-diffused stoichiometric LT is that the crystal maintains its transparency even after diffusion at a pressure as low as 0.1 Torr, thus without a post-annealing step required. The diffusion coefficient varied from D = 1.1 X 10(-2) to 5.5 x 10(-1) mum(2)/h in the given temperature range, with an activation energy of DeltaE = 1.95 +/- 0.3 eV. (C) 2004 Elsevier B.V. All rights reserved.