Injection-moulded semicrystalline nylon 6 and amorphous nylon 6IcoT blends were homogeneous (single glass transition, T-g) but metastable. Annealing above T-g increased crystallinity and induced phase separation probably due to crystallization and differences in polymer stiffness and spatial geometry. Crystallization of the nylon 6 component was greater than that of the pure nylon 6 in some blends since dilution with nylon 6IcoT would have decreased the number of tie molecules and increased the mobility of nylon 6 molecules. Microcrystals formed during annealing and their melting temperature and melting energy depended on composition. There was no evidence for significant transamidization between nylon 6 and nylon 6IcoT during injection moulding and annealing.