Copolymerizing poly(hexamethylene terephthalamide-co-isophthalamide) (PA6TI) with 40 wt% polyamide 66 (PA66) results in only minor decreases in modulus and yield stress owing to isomorphous substitution of PA6TI units by PA66 units in the PA6TI alpha phase, so that crystallinity is maintained in the resulting PA6TI-66 terpolymer. However, the ductility also significantly increases. This is attributed to a deformation-induced transformation of the PA6TI alpha phase to a disordered form of the PA66 alpha phase, where PA6TI units constitute defects and the chains adopt extended conformations. This phase is argued to be favored in initially amorphous PA66-rich regions and hence stabilizes interlamellar micro-necks that develop during yielding. By contrast, segregation of isophthaloyl units to the amorphous regions in PA6TI hinders strain-induced crystallization, leading to failure at significantly lower strains. Brittleness may therefore be a generic problem when comonomers ar e used to facilitate processing , but one that may be overcome through proper microstructural control.