The optical damage behaviour of different LiNbO3 optical waveguides has been experimentally studied by measuring the intensity output of a single beam as a function of the intensity input. Parallel measurements of photovoltaic currents have been carried out as a function of the input intensity and they have been correlated with the optical damage data. The following LiNbO3 guides have been studied and compared: proton exchanged (PE) belonging to the phases alpha, beta1, beta2 and reverse proton exchanged (RPE), and Zn in-diffused waveguides. The greatest intensity thresholds for optical damage, about 2x103 times greater than that of the substrate, have been obtained in RPE guides (they support ordinary polarization and have similar nonlinear optic activity as the substrate) and beta2 guides which support extraordinary polarization (they have no nonlinear optic activity). On the other hand, the lowest photovoltaic currents have been measured in beta1,2-phases. As a function of the light intensity, the photovoltaic current exhibits a superlinear behaviour, strong in alpha-phase and weaker in Zn in-diffused and RPE guides. The results for optical damage are discussed in connection with those of photovoltaic currents, paying particular attention to the main mechanisms involved.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.