Abstract Quenching and Partitioning (Q&P) is a new concept in martensitic steel heat treatment which, as well as other opportunities, offers potential for expansion of the elongation versus strength envelope available to steel users, especially the automotive industry. The Q&P steel concept shares similarities with TRIP steel in that both promote multiphase microstructures of stabilised retained austenite and a harder phase of bainite (TRIP) or martensite (Q&P), although in Q&P the two stages of martensite formation and austenite stabilisation by carbon partitioning are separated. The procedure also provides for both protection and temper strengthening of the martensite fraction. However, confirmatory study of reaction mechanisms has been frustrated by the elevated temperatures required to date to apply Q&P heat treatment to commercial steel compositions. Consequently, a model alloy has been used to separate the various stages of the Q&P heat treatment process, thus allowing diffraction experiments, particularly real-time in situ neutron diffractometry, to measure lattice parameter, lattice strain and phase fraction in order to deduce the concentration and distribution of carbon at different stages during the Q&P treatment.