The microstructure following a new martensite heat treatment has been examined, principally by high-resolution microanalytical transmission electron microscopy and by atom probe tomography. The new process involves quenching to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by ageing either at or above, the initial quench temperature, whereupon carbon can partition from the supersaturated martensite phase to the untransformed austenite phase. Thus the treatment has been termed ‘Quenching and Partitioning’ (Q&P). The carbon must be protected from competing reactions, primarily carbide precipitation, during the first quench and partitioning steps, thus enabling the untransformed austenite to be enriched in carbon and largely stabilised against further decomposition to martensite upon final quenching to room temperature. This microstructural objective is almost directly opposed to conventional quenching and tempering of martensite, which seeks to eliminate retained austenite and where carbon supersaturation is relieved by carbide precipitation. This study focuses upon a steel composition representative of a TRIP-assisted sheet steel. The Q&P microstructure is characterised, paying particular attention to the prospect for controlling or suppressing carbide precipitation by alloying, through examination of the carbide precipitation that occurs.