Resonant ultrasound spectroscopy and Transient Grating Spectroscopy were used for characterization of the diffusion controlled β→ ω phase transition in metastable β-Ti alloy Ti-15Mo. Ultrasonic methods are suitable tool for monitoring of β→ ω phase transition, because elastic constants of metastable β-Ti alloys are significantly influenced by formation of isothermal ω particles. Single crystals of metastable β-phase of the Ti-15Mo alloy with different heat treatments (isothermal/anisothermal ageing at different temperatures, ageing under uniaxial  loading) were studied with aim to observe dependence of elastic constants on formation of isothermal ω particles. Resonant ultrasound spectroscopy was used for observation of the temperature evolution of the elastic constants and Transient grating spectroscopy for determination of the local material symmetry at a fixed temperature. The results of our measurements show that the isothermal ω particles exactly follow the original cubic symmetry of the β matrix.