The decomposition of the b phase during rapid cooling of the near b titanium alloy Ti-5Al-5Mo-5V-3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic x structures form by the correlation of linear (1 1 1)b defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal x structures within the b matrix at temperatures $500 °C. Post-quench aging at 570 °C resulted in the nucleation of a laths after $90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of a precipitates with a lath morphology $1.5 lm  0.2 lm in size composed of both the expected Burgers variants. Mechanical property data suggests that the x structures alone have no real effect; however, hardness increases were observed as the a phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the a phase in near b titanium alloys and hence control mechanical properties.