The crystal structures of the odd-odd nylons X 11 (X = 3, 5, 7, 9, and 11) under different conditions, including solution-crystallized samples and melt-crystallized samples, were investigated by wide-angle X-ray diffraction (WAXD) and electron diffraction (ED). For the melt-crystallized samples of the odd-odd nylons under consideration, the strong diffraction at 0.42 nm, which is typical of the hexagonal lattice, was found in the WAXD patterns. As for the solution-crystallized samples, nylon 11 presents the monoclinic crystal structure characteristic of the two strong diffractions at about 0.42 and 0.40 nm of the WAXD and ED patterns, while nylons 9 11, 7 11, 5 11, and 3 11 show the hexagonal crystal ones typical of the same diffraction patterns as those of the melt-crystallized samples. The model of hydrogen bonds in crystal structures of the odd-odd nylons under study is discussed.