Abstract In recent works, we have demonstrated both experimentally and theoretically the occurrence of strong pressure shocks due to Rapid Phase Transition of water produced by combustion, if oxygen-enriched fuel mixtures are exploded in a non-adiabatic reactor. Indeed, peaks of hundreds of bars, although with very short duration, are observed if ultra-high-frequency acquisition system is adopted. In this work, experimental tests were performed, in a non-adiabatic 5 dm 3 cylindrical vessel, for methane explosion in oxygen varying fuel concentration and inert concentration and type (CO 2 , N 2 ). Both flammability limits, following the classical definition, and limits of fuel concentration for the combustion-induced Rapid Phase Transition (cRPT) phenomenon have been found experimentally and justified theoretically. Results are relevant to a range of applications and surely to mitigation/prevention measures. Reconsidering fuel hazards in real process is also mandatory.