During thermo-mechanical processing of metastable β Ti alloys such as Ti-5553, a variety of phase transformations can occur, including the precipitation of α-Ti, the ω-phase, the formation of silicides and, depending upon composition, the β-Ti may phase-separate into two β-Ti solid solutions. While the size and distribution of the phases resulting from these precipitation reactions can have a significant impact upon the properties, the influence of time and temperature upon the resulting microstructure is not well understood. Indeed, detailed TTT and CCT diagrams are not available for materials and design engineers. Hence, risk-free applications of these alloys in commercial systems remain limited. Using a suite of thermomechanical simulators, the microstructural evolution of Timetal 5553 has been studied, and the importance of the β-phase separation probed. This has led not only to a preliminary TTT diagram, but also to the development of databases relating microstructure and property in this important high-strength alloy.