Secondary-ion mass spectrometry (SIMS) was used to study the profile characteristics and diffusion properties of Mg, Ti, and Er ions in photorefractive-damage-resistant locally Er/Mg-diffused near-stoichiometric (NS) Ti:Mg:Er:LiNbO3 strip waveguides fabricated on two Z-cut initially congruent, undoped LiNbO3 substrates in sequence by local Er doping at 1100 °C or 1130 °C in air, Mg/Ti pre-diffusion at 1100 °C in wet O2, and post Li-rich vapor transport equilibration (VTE) treatment at 1100 °C. For comparison, a SIMS study was also carried out on the waveguides fabricated without the post-VTE treatment. In order to compensate for the refractive index decrease arising from both the Mg doping and the post-VTE treatment, and hence to get a positive net index increment profile in the Ti-diffused layer, a thicker Ti-film of around 170 nm was coated. Nevertheless, SIMS results show that the Ti diffusion reservoir, as well as the Er and Mg reservoirs, was exhausted. From the SIMS profiles, characteristic diffusion parameters such as the 1/e diffusion width (for Ti only) and depth, diffusivity, and surface concentration of the Mg, Ti, and Er ions are obtained. It is interesting that the Mg distribution in the NS waveguide layer is desirably homogeneous with a concentration [(1.7–2.0) ± 0.3 mol%] higher than the optical damage concentration threshold. The Ti profile follows a sum of two error functions along the lateral direction of NS waveguides with a diffusion width of (12–13) ± 0.5 μm, and a Gaussian function in the depth direction with a 1/e depth of (5.1–6.0) ± 0.2 μm. The Er profile follows also a Gaussian function with a 1/e depth of (3.7–4.4) ± 0.3 μm. In the NS waveguide layer, the mean diffusivity is (7.1 ± 2.2) to (8.3 ± 2.8) μm2/h for Mg, (3.5 ± 0.3) to (4.5 ± 0.4) μm2/h in the lateral direction and (0.54 ± 0.04) to (1.13 ± 0.08) μm2/h in the depth direction for Ti, and (4.1 ± 0.4) to (5.5 ± 0.5) × 10−2 μm2/h for Er. The effects of Li outward diffusion in the initial Er doping procedure, and the Mg codiffusion and post-VTE treatment on the mean Mg, Ti, and Er diffusivities are discussed in comparison with the previously reported results on single Mg, Ti, or Er diffusion and Mg/Ti codiffusion in a pure or homogeneously MgO-doped congruent LiNbO3 crystal. Finally, the keys to the success of the fabrication procedure adopted are discussed.