The results of a study of the decomposition of a metastable β-phase during aging of the alloys Ti–10V–2Fe–3Al, VST5553 (Ti–5Al–5V–5Mo–3Cr–0.5Fe), and Beta-21S (Ti–3Al–15Mo–2.7Nb–0.2Si, wt %) have been presented. A comparative study of the evolution of the microstructure and changes in the periods of the crystal lattice of phases in alloys during aging has been performed. Scanning electron microscopy has been used to study the morphology of the secondary α-phase formed during aging. The corresponding diffusion redistribution of alloying elements between the phases has been estimated using full-profile X‑ray diffraction analysis and thermodynamic modeling via a JMatPRO software code and numerical calculations. It has been established that during aging, the change in the periods of crystal lattices in the studied alloys exhibits a general pattern due to the same origin of the diffusion redistribution of alloying elements between phases.