The effects of blend composition and take-up velocity on structural variations and physical properties of poly(ethylene terephthalate) [PET]/poly(ethylene-2,6-naphthalate) [PEN] blend fibres were studied using WAXS, n.m.r., d.s.c., density, tensile and thermal shrinkage tests. Over the ranges studied, the structure and properties of the blend fibres were significantly influenced by the composition and melt spinning velocity. With the increase in content of any second component (PEN or PET), the crystallization process in the as-spun fibres becomes difficult, because of the formation of random copolymers via a transesterification reaction occurring between PET and PEN. molecules. This reaction also produces a miscible amorphous phase with the result that all blend samples exhibit a single glass transition temperature intermediate to those of the two homopolymers. The WAXS results of annealed samples, however, indicate separate crystals for the two components rather than a co-crystallization. Room temperature mechanical properties, for fibres spun at constant take-up velocity, improved gradually with increasing PEN content. The thermal shrinkage, however, is relatively high for blend fibres and reaches a maximum for the 50/50 PET/PEN blend, indicating that the copolymer chains are oriented fairly well, but exist in the amorphous state.