The crystal structures of poly(nonamethyleneoxamide) (nylon-9,2) were investigated by wide-angle Xray diffraction (WAXD) and differential scanning calorimetry. Nylon-9,2 shows a polymorphism with two different structures depending on the conditions of sample preparation. Monoclinic form I is the dominant crystalline phase for the solution-crystallized sample. For melt-crystallized samples, form I and new monoclinic form I' coexist. Variable-temperature WAXD measurements were used to investigate the stability of each crystal form and the phase transition. Form I does not undergo any phase transitions prior to melting, although the orthorhombic molecular chain packing in form I at low temperature comes close to the quasi-hexagonal one during the heating process. Form I' shows no phase transition on heating until its melting, too. The crystal lattice simply expands due to the thermal expansion. Form I is believed to be more stable, because the melting temperature of form I is higher than that of form I'. (C) 2014 Elsevier Ltd. All rights reserved.