The adsorption of dimethyl methylphosphonate (DMMP) on alumina-supported iron oxide has been examined. DMMP reacts on adsorption at room temperature, apparently through cleavage of the phosphorus-carbon bond. This bond is observed to be extremely resistant to cleavage when DMMP is adsorbed on oxides such as alumina, magnesia, and lanthana. The phosphorus-methoxy bonds, which are the most readily cleaved on the other oxides, appear at least initially to remain intact on the alumina-supported iron oxide. The hypothesis proposed to account for the unusual activity of the iron oxide surface is an oxidation pathway involving the Fe(II)/Fe(III) redox couple.