Mixtures of energetic compounds pose a remediation problem for munitions-contaminated soil. Although treatment with zerovalent iron (Fe0) can be effective, RDX and TNT are more readily destroyed than HMX. Adding didecyldimethylammonium bromide (didecyl) at 2% w/v with 3% (w/v) Fe0 to a 20% slurry of Los Alamos National Laboratory soil containing solid-phase HMX (45 000 mg/kg) resulted in >80% destruction within 6 days. Because the HMX concentration did not increase in solution and the didecyl equilibrium concentration was well below the critical micelle concentration, we conclude thatthe solution primarily contained didecyl monomers. The adsorption isotherm for didecyl on iron is consistent with electrostatic adsorption of monomers and some hydrophobic partitioning at low equilibrium concentrations. Fe0 pretreated with didecyl was superior to Fe0 alone or mixed with didecyl in removing HMX from solution, but it was less effective than Fe0 + didecyl when solid-phase HMX was present. Reseeding HMX to mimic dissolution indicated an initial high reactivity of didecyl-pretreated Fe0, but the reaction slowed with each HMX addition. In contrast, reaction rates were lower but reactivity was maintained when Fe0 and didecyl were added together and didecyl was included in fresh HMX solutions. Destruction of solid-phase HMX requires low didecyl concentrations in solution so that hydrophobic patches are maintained on the iron surface.