Samples of various iron oxides suspended above ethylene oxide in an adiabatic calorimeter exhibit exothermic activity at temperatures as low as room temperature. A gamma-Fe2O3 sample was found to show the highest reactivity with ethylene oxide. Ethylene oxide in combination with most of the iron oxide fines tested displayed exothermic activity below 100 degreesC. Self-heat rates near 2000 degreesC/min were observed for the gamma-Fe2O3 fines while rates in excess of 100 degreesC/min were found for other fines (alpha-Fe2O3 and hydrated alpha-Fe2O3). In two cases (alpha-Fe3O4 and alpha-Fe2O3), pressurization rates above 1000 psi/min took place. No reactivity was observed for ethylene oxide with the FeO. Thermal inertia effects in commercial operation, such as heat uptake by the equipment to which fines are attached, are presumed to be a factor in limiting the occurrence of related exotherms in ethylene oxide manufacturing facilities. (C) 2003 Shell International Chemicals B.V. Published by Elsevier B.V.