Fabrication of optical waveguides on LiNbO3by Zn diffusion from the vapor phase has been demonstrated, using a specially designed diffusion apparatus. The planar waveguides onz‐cut samples support both TE and TM propagations, indicating an increase of refractive indices forn0andne. Energy dispersive x‐ray analysis measurements confirm the diffusion of Zn into the crystal, and analysis of the content of Zn as a function of diffusion depth for samples treated at different temperatures indicates much higher diffusion coefficients than those obtained for Ti diffusion in LiNbO3. The technique is versatile, easy to operate, and allows fabrication of waveguides at temperatures as low as 600–700 °C, thus preventing Li outdiffusion. ©1995 American Institute of Physics.