The crystalline polyamide phase in two representative poly(ether-block-amide) copolymers has been shown to be polymorphic in nature via an analysis by X-ray diffraction and solid-state C-13 nuclear magnetic resonance. In both polymers, this phase was found to adopt alpha and gamma structures analogous to those that exist in the corresponding polyamide homopolymers. Furthermore, it has been shown that phase transformation between crystal types is possible using chemical modifications.