A method is described for producing high quality optical waveguides on Z-cut LiNbO3 substrates by proton exchange in pure benzoic acid vapor. Either α- or β-phase guides are obtained by varying the exchange temperature (within the 250–375 °C range) and/or the treatment duration (between 15 min and 24 h). The method presents definitive advantages with regards to common immersion methods. For both α- and β-phase guides the d33 nonlinear optical coefficient is 90% of the substrate value, and their optical losses are below 0.35 dB/cm. This is the first report on β-phase guides with almost undegraded nonlinear optical coefficients. © 1997 American Institute of Physics.