The quenching and partitioning (Q&P) process is a heat treatment process, aiming at the creation of steel microstructures composed of martensite and retained austenite. Herein, the thermal stability of the microstructure is investigated upon reheating steel microstructures, created with different Q&P settings, in different thermal routes, using dilatometry and thermomagnetometry to quantitatively monitor phase fractions. Analysis of the derivative of dilatometry curves and thermomagnetic data reveals that upon reheating the retained austenite decomposes. The decomposition occurs in two stages when the heating rate is relatively low. The retained austenite completely transforms to ferrite and cementite upon reheating to 550 °C, followed by isothermal holding for 1800 s. Increasing the partitioning time from 50 to 300 s at 400 °C after quenching to 260 °C significantly increases the thermal stability of retained austenite. In all conditions, both carbon-depleted martensite (formed in the initial quenching step) and fresh martensite (formed in final Q&P quenching) are found to be partially tempered during the reheating experiments.