Samples of Ti-15Cr and Ti-15V-3Sn-3Al-3Cr (wt%) containing controlled additions of carbon up to 0.2wt% and different oxygen contents have been quenched and aged at temperatures between 400 and 600°C. Optical, scanning and analytical transmission electron microscopy have been used to characterise the microstructures of the quenched and aged samples. Hardness testing has been used to follow the kinetics and extent of age hardening, which are accelerated in C-containing samples. The addition of carbon results in the formation of Ti(CxOy) precipitates which pin grain boundaries in forged samples so that the grain size in the quenched C-containing samples is about a factor of ten less than that in the C-free samples. In the C-free samples coarse grain boundary alpha tends to form, but in the C-containing samples alpha precipitation is more uniform throughout the beta grains. The extent of omega precipitation is very different in the two alloys; the Ti-15Cr alloy forms athermal omega in the as-quenched samples and large omega precipitates are formed on ageing at 40C No evidence for omega has been obtained in the Ti-15-3. The hardening responses and microstructural observations are interpreted in terms of the different grain boundary oxygen contents in the C-containing and C-free samples and the different roles of omega and of carbon in the two alloys.