Corrosion and passivation of solution-treated (ST), solution-treated and aged (STA), and diffusion-hardened (DH) variants of Ti-13% Nb-13% Zr (Ti-13-13) were examined in a simulated physiological (lactated Ringer's) and simulated occluded cell (5 M hydrochloric acid [HCl]) environment. Commercially pure (CP) Ti (Grade 2), Ti-6% Al-4% V, Ti-35% Zr-10% Nb, ST and STA Ti-15% Mo-3% Nb-3% Al (Ti-15-3-3), and Ti-55% Ni also were tested. In the Ringer's solution, the Ti-based alloys all exhibited spontaneous passivity and similar passive current densities (ip∼ 5 × 10−7 A/cm2). The Ti-based implant alloys exhibited active-to-passive transitions indicative of incomplete passivity resulting from the stability of the titanium anion (Ti+) or titanium chloride anion (TiCl2+) at deaerated open-circuit potentials (OCP) in 5 M HCl. In contrast DH Ti-13-13 exhibited spontaneous passivity in 5 M HCl as a result of its 0.8-μm-thick thermal oxide. Anodic polarization and mass-loss investigations of Ti-Nb, Ti-Zr, Ti-Mo, and Ti-Nb-Zr-based alloys in 5 M HCl revealed that additions of Nb, Zr, and Mo improved the corrosion resistance and passivity of Ti, with spontaneous passivity possible with sufficient alloying. The inferior corrosion behavior of STA Ti-15-3-3, with β + α microstructure, in comparison to ST Ti-15-3-3, with a single phase β solid solution microstructure, was attributed in part to β and α stabilizer partitioning during the metastable β (body-centered cubic [bcc]) → β + α phase transformation, which resulted in an Al-rich α phase and a Mo- and Nb-rich β phase in the aged microstructure. This was confirmed by anodic polarization experiments of Ti-Al binary alloys, as well as by energy dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) nanostructural analysis of corroded STA alloy surfaces. Isothermal aging of Ti-13-13 resulted in the partial phase transformation from metastable β and diffusionless α′ (martensitic hexagonal close-packed [hcp]) to a β + α′ + α microstructure. The slight partitioning of Nb into the β phase, the presence of significant concentrations of Zr in the β and α phases, and lack of Al accounted for the improved passivity and corrosion resistance of STA Ti-13-13 compared to STA Ti-15-3-3.