The tensile modulus, tensile strength and impact strength of melt blends of (a) poly(ethylene naphthalate) (PEN) and poly(butylene terephalate) (PBT) with 30, 40, 50, 60 and 70 wt% PEN, (b) Nylon66 and PEN with 30, 50 and 70 wt% Nylon66 were measured, and thermal/thermomechanical properties were analysed by differential scanning calorimetry and dynamic mechanical thermal analysis. Scanning electron microscopy was used for examination of the fracture surfaces of the blends.All PBT/PEN blends show two glass transitions corresponding to the presence of two phases: the glass transition temperature, T-g, of the phase with the lower Tg increases with increasing PEN content, and T-g for the phase with higher T-g decreases with increasing PBT content. The implication is that the two polymers are partially miscible, and scanning electron microscopy of fracture surfaces reveals a very small (sub-micron) domain size. Nylon66/PEN blends also show two phases, but the domain size is of the order of mum and there is no evidence of partial miscibility.Up to 50 weight proportions PBT does not lower the tensile strength of PBT/PEN blends, and the tensile strength lies between values predicted by the rule of mixtures and a modified rule of mixtures. Incorporation of at least 40% PEN in PBT increases impact strength, but blending with smaller proportions of PEN decreases impact strength. By contrast, blending of Ny66 and PEN results in reduction of tensile strength for all blend compositions. (C) 2003 Kluwer Academic Publishers.