The morphology of a series of melt-miscible blends based on the combination of (crystalline) polyamide (PA) 4,6 and (amorphous) PA 61 has been investigated as a function of blend composition and thermal history. Wide-angle X-ray scattering (WAXS) was used to determine the unit cell structure and crystallinity index in PA 4,6 and the blends: the introduction of PA 6I resulted in no change in unit cell parameters and a monotonic decrease in bulk crystallinity. Dynamic mechanical studies revealed the coexistence of two amorphous phases in the crystallized blends: a mixed phase held between the crystal lamellae (interlamellar) and a PA 6I-rich phase excluded to interfibrillar and interspherulitic regions. There was no evidence of a separate crystal-amorphous interphase layer comprised of pure PA 4,6. Transamidation reactions over longer melt times led to the apparent homogenization of the two amorphous populations owing to the formation of increasingly random copolymers. (C) 2000 Elsevier Science Ltd. All rights reserved.