The adsorption and decomposition reactions of dimethyl methylphosphonate (DMMP) on cerium and iron oxides supported on aluminum oxide have been examined at 25 degreesC. The capacities of these solids for the decomposition of DMMP have been measured, and the identities and amounts of the decomposition products determined. The coimpregnated oxide formulations are significantly more reactive than alumina alone, and the current formulations are 2.5x more reactive at room temperature than any other metal oxide studied previously. A series of screening experiments show that the most active formulation is one containing 5 wt % iron and 7.5 wt % cerium. At 25 degreesC, Al2O3 shows a decomposition capacity of 317 mumol/g, while the alumina-supported iron and cerium oxide combination shows a decomposition capacity of more than 510 mumol/g. Formulations containing similar amounts of iron oxide or cerium oxide individually are more active than the unmodified alumina but less active than the coimpregnated oxide. The results show that the three-dimensional CeO2 phase that forms when cerium oxide is impregnated on alumina by itself is inactive for decomposition, and that the increased reactivity for these materials originates with a two-dimensional cerium oxide phase. The increased activity for the materials that include iron is suggested to be due to the iron either increasing the number of defect sites in the ceria crystallites that do form, or facilitating the formation of smaller crystallites.