When exploding CH4/O2/N2 mixtures with high oxygen contents in a nonadiabatic vessel, the pressuretime histories display oscillations of different frequencies and very high pressure peaks (hundreds of bars). We have attributed this anomalous behavior (combustion-induced rapid phase transition, cRPT) to the occurrence of cycles of condensation/evaporation of water at the vessel walls, followed by superheating of the liquid film due to radiative heat transfer from the flame, which culminates in the water rapid phase transition. We now report a detailed analysis of the role played by the addition of a diluent (CO2, N2, He, and Ar) on the anomalous behavior. The limit values of the diluent concentration at which the cRPT phenomenon is suppressed have been found and correlated to the kinematic viscosity and the thermal diffusivity through the Prandtl number. The less effective diluent has been found to be Ar followed by He, N2, and CO2 in the order listed. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012