Thanks to time resolved high energy X-ray diffraction, isothermal decomposition of α" metastable phase was studied, directly after solution treatment in the β temperature range, for temperatures ranging from 300 to 450°C for two β metastable alloys (Ti 17 and Ti 5553). The formation of an orthorhombic α" phase is clearly identified at the beginning of the transformation whatever the alloy studied. If transformation occurs at the higher temperature an evolution of α" is observed toward the hexagonal α phase. The phase amounts and the mean cell parameters of each phase were quantified by the Rietveld refinement method. The obtained cell parameters evolutions and the orthorhombicity of α" are discussed. Moreover, the orthorhombicity of α" compared to that obtained for stress induced martensite may indicate a slight partitioning of solutes in isothermal α".