A structural study is presented on proton exchanged LiNbO3 waveguides prepared in benzoic acid vapor which, for z-cut substrates, were previously reported to simultaneously exhibit high index jump, very low losses, and undegraded nonlinear optical coefficients. X-ray rocking curves of z-cut guides show a unique, intense peak with the same bandwidth as the substrate peak, located at the κ2-phase position of HxLi1-xNbO3, as well as a small α-phase contribution. For x-cut guides only the β1-phase peak is obtained, with a bandwidth about 60% greater than the substrate peak. Distinctive features of κ2 and β1 guides are also shown in micro-Raman spectra. © 1997 American Institute of Physics.