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Metastable β-Ti alloys have become one of the most attractive implant materials due to their high biocorrosion resistance, biocompatibility, and mechanical properties, including lower Young's modulus values. Mechanical properties of these alloys are strongly dependent on the final microstructure, which is controlled by thermomechanical treatment processing, in particular the Young's modulus and hardness. The aim of this work was to analyze the influence of phase precipitations in heat-treated Ti-12Mo-xNb (x = 0, 3, 8, 13, 17, and 20) alloys. The alloys were prepared via arc melting and treated at 950 °C/1 h, and then quenched in water. The microstructures were analyzed by optical microscopy, transmission electron microscopy, and X-ray diffraction. Mechanical properties were based on Vickers microhardness tests and Young's modulus measurements. Microstructural characterization showed that α″ and ω stability is a function of Nb content for the Ti–12Mo base alloy. Nb addition resulted in the suppression of the α″ phase and decrease in the ω phase volume fraction. Although the ω phase decreased with higher Nb contents, ω particles with ellipsoidal morphology were still observed in the Ti–12Mo–20Nb alloy. The α″ phase suppression by Nb addition caused a marked increase in the Young's modulus, which decreased back to lower values with higher Nb concentrations. On other hand, the decrease in the ω phase continuously reduced alloy hardness. The study of the effect of chemical composition in controlling the volume fraction of these phases is an important step for the development of β-Ti alloys with functional properties. [ABSTRACT FROM AUTHOR]