Nylon-11 polymorphs have been studied by x-ray diffraction and by infrared spectroscopy. A gamma or gamma-like phase of Nylon-11 was produced by treating the metastable delta’ phase with gaseous HCl or DCl and then exposing the resulting complex (amidonium salt) to 100% RH H2O or D2O vapor. An oriented form of the gamma or gamma-like phase results from treatment of the oriented delta’ phase, but stretching of the unoriented gamma or gamma-like phase, obtained by treatment of the unoriented delta’ phase, induces a transformation to the ct phase stable at room temperature. The use of DCl and D2O produced highly deuterated samples with Fermi resonances indicating the relative strengths of the hydrogen bonds are weakest in the a phase, intermediate in the delta’ phase and strongest in the gamma or gamma-like phase.