The structure of nylon 5, 10 has been investigated using electron microscopy and X-ray diffraction. Nylon 5, 10 shows polymorphism with two different structures related to the gamma form obtained by either solution or melt crystallization. Packing differences may be attributed to a change in the hydrogen bonding system. In addition, a structure related to an alpha-like phase can be found by precipitation from strong acid solutions and their mixtures with chloroform. A model with two hydrogen bonding directions is given for this form, an a similar way to that recently postulated for polyamides derived from odd diamine or odd diacid units. Temperature-induced structural changes have also been studied for nylon 5, 10 fibres.