Explosion tests were performed for CH4/O-2/N-2 mixtures in a non-adiabatic 5 L cylindrical vessel. Experiments were run for mixtures with stoichiometric CH4/O-2 ratio, changing the oxygen air enrichment factor, E = O-2/(O-2 + N-2), from 0.21 (air) to 1 (pure oxygen). Oscillating pressure time histories with over-adiabatic peaks (up to 240 bar) have been measured. The oscillating behaviors can be attributed to the occurrence of cycles of condensation and vaporization of the water produced during combustion. Such cycles culminate in the rapid phase transition of the condensed water, leading to the over-adiabatic pressure peaks. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.