Copolyamides, based on 1,12-dodecanedicarboxylic acid and different ratios of 1,2-ethylenediamine and piperazine, i.e., PA-(2.14-co-pip.14), as well as the corresponding homopolymers PA-2.14 and PA-pip.14, were studied by SAXS and WAXS. Up to a pip mole ratio of 0.62, the 2.14 and pip. 14 units cocrystallize in a common crystal lattice, slightly deviating from the structure of homopolyamide 2.14. The hydrogen bonds obviously tolerate significant amounts of comonomer before the crystal structure is changed significantly. For pip mole percentages of 0.90 and higher, both repeating units cocrystallize in a slightly distorted PA-pip.14 crystal structure. For pip mole percentages of 0.70 and 0.82, however, the X-ray patterns show peaks that stem from both the PA-2.14- and the PA-pip.14-like crystalline structure, indicating that the two structures coexist in this composition range. Since the intersheet distance practically remains unaffected upon incorporation of the piperazine rings, it is concluded that these rings are oriented parallel to the hydrogen-bonded sheets. Furthermore, from the composition dependency of the experimentally determined lattice spacings, keeping in mind that the intersheet distance is constant, it is concluded that the hydrogen-bonded sheets are shifted parallel to one another. © 2005 American Chemical Society.