Microstructures in a commercial Nb-Ti alloy heat-treated or repeatedly aged and cold worked have been investigated by transmission electron microscopy. In the aged samples after solution treatment,ω-phase forms at ageing temperatures below 673 K andα-phase precipitates at 773 K. Cold work after solution treatment or heavy cold work without prior solution treatment results in accelerated precipitation ofω-phase on ageing at 623 K and ofα-phase on ageing at 773 K, while it givesα-precipitation instead ofω on ageing at 673 K. Repeated ageing and cold work make the microstructures very fine. The refinement is due to formation of dislocation cell structures and subgrains and precipitation ofω- andα-phases.