Relationships were found between the heats of fusion Delta H-m,H-tr of 12 nitramines with rigid molecules, on one hand, and their molecular weights or their lengths of the longest N-N bonds or N-15 NMR chemical shifts of their amine and/or nitro group nitrogen atoms, on the other hand. On the basis of these relationships, the Delta H-m,H-tr values have been predicted for 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW, Delta H-m,H-tr - 42.70 kJ mol(-1)) and 1,10-dinitro-2,6,8,12-tetraoxa-4,10-diazaisowurtzitane (TEX, Delta H-m,H-tr = 36.10 kJ mol(-1)). It was stated that the HNIW molecule contributes to the intermolecular potential in its molecular crystals mainly by its nitro groups at 2, 6, 8, and 12 positions. The N-N bond length of 0.1377 nm was also predicted for nitramino groupings in 1 and 5 positions of the 1,3,5-trinitro-1,3,5-triazacycloheptane molecule.