The linear secondary nitramine, 1,7-dimethyl-1,3,5,7-tetranitrotrimethylenetetramine CH 3[N(NO 2)CH 2] 3N(NO 2)CH 3 (OHMX) and its cyclic analog, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, (CH 2NNO 2) 4 (HMX) are compared. By IR spectroscopy and DTA, OHMX is found to exist in five polymorphs between 273 K and its melting point at 490 K. The IR spectra suggest that the structural differences involve mostly the NO 2 and CH 3 groups and changes in lattice symmetry without a significant alteration in the conformation of the backbone. The kinetic parameters for the OHMX(III → IV) phase transition are first order ( E a = 50.8 kcal mol −1) and are comparable to those of HMX. The nucleation process is probably being measured. The values of Δ H for the phase transitions of OHMX (0.8–2.1 kcal mol −1) are also similar to those of HMX. The molecular structure of OHMX(II) was determined: monoclinic, P2 1/ c, a = 20.952(7), b = 9.477(2), c = 6.521(2) Å, β = 96.29(3)°, Z = 4, D calc = 1.598 g cm −3, R F = 3.84, R wF = 4.68%. It consists of a helical chain of CH 2NNO 2 units staggered at approximately 120° intervals. A pseudo-C 2 axis is present. OHMX is slightly less thermally stable than HMX. High-rate thermolysis (d T /d t =110 K s −1) of OHMX liberates products that are the same, with a few exceptions, as for HMX. Competitive N-N and C-N bonds fission occurs as predicted by the IR data and molecular structure. The pressure dependence of the decomposition process indicates that the heterogeneous gas-condensed phase reactions occur to a greater extent as the pressure increases.