The mutual influence of co-diffused Ti4+ and Sc3+ in LiNbO3 single-crystal was studied. Sc3+/Ti4+ co-doped LN plates were prepared by annealing (at 1060 degrees C in wet O-2) Z-cut congruent LiNbO3 substrates coated with stacked Sc2O3 and Ti films, which have different thicknesses and coating sequences. After the annealing, the refractive index and composition at the undoped and Sc3+-only doped surface parts were characterized by prism coupling technique. The Sc3+ and Ti4+ profiles were analyzed by secondary ion mass spectrometry. The results show that the Sc3+-doping has little contribution to the LiNbO3 refractive index. Li2O out-diffusion is not measurable. In the only diffusion case, the Sc3+ diffuses five times slower than the Ti4+. In the Sc3+/Ti4+ co-diffusion case, the Ti4+ assists and controls the Sc3+ diffusion. The Sc3+ diffusivity increases linearly with the initial Ti-metal thickness. In contrast, the Sc3+ affects the Ti4+ diffusion little. Neither the Sc2O3 film thickness nor the coating sequence influences the diffusivity of the two ions. The co-diffusion features are explained. A model for the co-diffusion system is suggested and verified experimentally. The Ti:Sc:LiNbO3 planar and strip waveguide are optically characterized. The results show that the Sc3+ presence does not affect the waveguide characteristics and the waveguide is optical-damage-resistant.