Synchrotron X-ray diffraction and transmission electron microscopy experiments are employed to investigate the mechanical stability of retained austenite in a quenching and partitioning steel. It is found that martensitic transformation takes place in high carbon blocky retained austenite at the onset of deformation, while low carbon film-like retained austenite, despite having a much lower carbon content, is stable at strains up to 12%. Two possible mechanisms are employed to explain these interesting findings.