Ti-containing micro-alloyed steels are often alloyed with molybdenum (Mo) to gain superior ageing-resistance through reduced nano-precipitate coarsening, although the exact mechanism is still disputed. Using small angle neutron scattering (SANS), we characterize precipitate composition and coarsening in Ti micro-alloyed steels with and without Mo through analysis of both the nuclear and magnetic signals. The SANS results are correlated to phase transformations, precipitation formation and mechanical properties of the steels. The SANS results demonstrate that~25 at.% of Ti is substituted by Mo in the (Ti, Mo)C precipitates, increasing both the precipitate volume percent (from ~0.1% to ~0.13%) and average size at a given ageing time. Our study reveals that Mo does not play a role in retarding coarsening of the precipitates, which is contrary to existing literature. The mechanism providing Ti-Mo steels with improved ageing-resistance is believed due to the improved misfit between precipitate and matrix, delaying loss of coherency. This new understanding opens opportunities for design of ageing-resistant microalloyed steels with use of significantly less amount of costly Mo.