The explosion behavior of CO/O-2/N-2 mixtures was experimentally studied in a tubular closed vessel at different oxygen contents, in the presence and in the absence of hydrogen. We found that CO/O-2/N-2 mixture explosion does not exhibit cRPT whatever the oxygen content thus demonstrating that this phenomenon is strictly connected to the water produced by the combustion reaction. Conversely, the addition of even very small amounts of hydrogen, triggers the cRPT phenomenon eventually leading to over-adiabatic peak pressures. The obtained results confirm the key role of water in driving the cRPT phenomenon. In the presence of hydrogen (even small amounts), an intense explosion, the combustion-induced Rapid Phase Transition (cRPT), was found. This highlights the key role of water in driving the cRPT phenomenon. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.