The Quenching and Partitioning (Q&P) process is a promising route for the development of new martensitic stainless steels with improved combinations of strength and ductility, in which the presence of retained austenite is believed to play an important role. This work investigates the mechanical stability of retained austenite in three martensitic stainless steels processed by Q&P. The volume fraction of retained austenite is measured after interrupted tensile tests through X-ray diffraction (XRD) experiments. Results show that the mechanically-induced formation of fresh martensite starts after yielding of the material. The presence of large islands of retained austenite seems to be linked to reduced mechanical stability, probably due to a less efficient carbon homogenization during the partitioning step. However, an intrinsic effect of the austenite grain size could also play a role. Therefore, it is suggested that increased mechanical stability is reached by microstructures in which the islands of retained austenite are reduced, and after partitioning steps sufficiently long to allow full carbon homogenization of the austenite.