QuasistoichiometricLiNbO3was used as substrate for proton exchanged waveguides prepared by immersion in hot benzoic acid for different times and melt compositions. Both &agr;- and &bgr;-phase guides were investigated. Their basic properties were studied and compared with those of guides prepared under the same conditions on congruent substrates. For &agr;-phase guides on quasistoichiometric substrates, the values of the index jump and the effective diffusion coefficient are, respectively, 0.53 and 0.54 times those obtained on congruent substrates under the same conditions. Also, theird33coefficient for second harmonic generation is about 10&percent; greater, while the optical damage resistance is a factor of 5 smaller. On the other hand, the properties exhibited by &bgr;-phase guides depend very little on the substrate stoichiometry. The results are discussed in terms of the lithium deficiency generated during the exchange in comparison to that previously existing in the substrate used. ©1997 American Institute of Physics.