We have studied the crystalline phase transition for vapor-phase proton exchanged (VPE) lithium niobate waveguide by using pure benzoic acid. The VPE layers in the z-cut lithium niobate crystal have been characterized by X-ray rocking curves, OH- bond infrared absorption, and prism coupling measurements. The crystalline quality assessment by the X-ray rocking curve measurement shows desirable kappa(2) phase formation for nonlinear optical waveguide applications. It has been observed that the crystalline phase makes a transition from the kappa(2) to the beta phase with the increase in the vapor-phase exchange time. The prism coupling measurement also showed a step-like index profile for our VPE samples. From the step index profile, we deduced an effective diffusion coefficient of 0.14 mum(2) h(-1), which is twice than that of the previously reported value. Shift in the OH-bond absorption wavelength with crystalline phase change and the increase in absorption with the exchange time were also observed from our spectrophotometer measurement. (C) 2002 Elsevier Science B.V. All rights reserved.