RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) are cyclic nitramines ((CH2NNO2)n; n = 3 or 4, respectively) widely used as energetic chemicals. Their extensive use led to wide environmental contamination. In contrast to RDX, HMX tends to accumulate in soils due to its unique recalcitrance. In the present study, we investigated the potential of zerovalent iron (ZVI) to transform HMX under anoxic conditions. HMX underwent a rapid transformation when added in well-mixed anoxic ZVI-H2O batch systems to ultimately produce formaldehyde (HCHO), ammonium (NH4+), hydrazine (NH2NH2), and nitrous oxide (N2O). Time course experiments showed that the mechanism of HMX transformation occurred through at least two initial reactions. One reaction involved the sequential reduction of N-NO2 groups to the five nitroso products (1NO-HMX, cis-2NO-HMX, trans-2NO-HMX, 3NO-HMX, and 4NO-HMX). Another implied ring cleavage from either HMX or 1NO-HMX as demonstrated by the observation of methylenedinitramine (NH(NO2)CH2NH(NO2)) and another intermediate that was tentatively identified as (NH(NO2)CH2N(NO)CH2NH(NO2)) or its isomer (NH(NO)CH2N(NO2)CH2NH(NO2)). This is the first study that demonstrates transformation of HMX by ZVI to significant amounts of NH2NH2 and HCHO. Both toxic products seemed to persist under reductive conditions, thereby suggesting that the ultimate fate of these chemicals, particularly hydrazine, should be understood prior to using zerovalent iron to remediate cyclic nitramines.