Some longer-chainbis-alkanamides of ethylenediamine have been synthesized and crystallized for use as potential standards for the measurement of decomposition of aminophylline suppositories. The thermal properties of the compounds have been investigated by differential scanning calorimetry (DSC). Each of the compounds was found to give two endotherms. The endotherms at the higher temperature corresponded to the melting temperatures. However, over the homologous series, the upper endotherm demonstrated paradoxical behavior in that the temperature decreased as the length of the alkanoyl chain, and hence the relative molecular mass, increased. The endotherm at the lower temperature could not be attributed to any visible change in the physical or microscopic state of the crystals. It behaved more as expected from the thermal properties of other fatty acid derivatives, such as the esters, mono-, di-, and triglycerides, in that the temperature increased as the alkanoyl chainlength increased. Cooling the compounds to 25°C after this first DSC run and then reheating under the same conditions showed that the upper endotherm remained the same but that a number of new endotherms replaced the lower endotherm. The endotherms found during this repeat run had lower enthalpies per gram than were obtained from the freshly crystallized materials. A possible structure for the crystals is presented. A model is proposed with hydrogen bonding and van der Waals bonding between adjacent molecules where differential breaking of these bonds causes the two initial endotherms