Er3+/Ti4+-co-doped LiNbO3 plates were prepared by annealing congruent substrates coated with stacked Er- and Ti-metal films having different thicknesses and coating sequences. The crystalline phase, OH- absorption, Er3+-doping effect on LiNbO3 refractive index and Li+-composition in the Er3+/Ti4+-co-doped layer were studied. The results show that the LiNbO3 phase dominates in the co-doped layer, the Er3+-doping affects little the LiNbO3 refractive index, slight Li2O out-diffusion occurred in the diffusion procedure, and the annealing procedure does not affect the OH- absorption band noticeably. The Er3+/Ti4+-co-diffusion characteristics were studied by secondary ion mass spectrometry. The study shows that Ti4+ diffuses much faster than Er3+. The Ti4+ co-diffusion assists and controls the Er3+-diffusion and Er3+ diffusivity increases linearly with initial Ti-metal thickness. In contrast, neither Er-metal thickness nor coating sequence of Ti- and Er-metal films influences the Ti4+ or Er3+ diffusivity. The co-diffusion features are explained. A co-diffusion model is proposed. (C) 2015 Elsevier Ltd. All rights reserved.