The fact that an explosion starts at a lower ambient temperature when methyl ethyl ketone peroxide (MEKPO) is not sufficiently neutralized, is demonstrated with a highly sensitive small-scale calorimeter (C80D). The data indicate that MEKPO might undergo acid and redox decompositions in addition to thermal decomposition in the accidental scenario. Meanwhile, experimental results of modified closed pressure vessel tests (MCPVT) suggest that the decomposition is much more active for MEKPO in the presence of sulfuric acid. Under such conditions, the maximum pressures and the maximum rates of pressure rise are significantly higher than those of pure MEKPO. (C) 2003 Elsevier Ltd. All rights reserved.