The temperature dependence of free volume in random copolyesters of hydroxybenzoic acid (HBA) and hydroxynaphthoic acid (HNA) has been studied from -50 to +350 °C by measuring the average lifetime, τ 3 , and intensity, I 3 , of ortho-positronium (o-Ps) annihilation. These parameters have also been used to determine the fractional free volume, h ps . Significant changes in the temperature coefficients of τ 3 , I 3 , and h ps are observed at the α-transition temperature, T α , and at the melt transition temperature, T m . The results indicate that both the average size and the number of free volume cavities occupied by positronium increase at T α , continue to increase uniformly with temperature up to T m , and then level off. A miscible blend of 75/25 and 30/70 copoly(HBA/HNA) with overall monomer ratio 60/40 exhibits a single melt transition well below those of the component copolymers or a random copolymer of the same composition, suggesting that the chains are less ordered in the blend. Comparison of PALS data for these systems indicates that the blend has a larger h ps below T m , which is primarily due to a larger number of cavities accessible to o-Ps, and a substantially larger temperature coefficient of h ps between T α and T m . In the nematic melt, the free volume of the blend decreases to that of the pure copolyesters, indicating that h ps in the nematic phase is independent of composition. Compared to other amorphous or semicrystalline polymers, the HBA/HNA copolymers have both fewer detectable free volume cavities and smaller average cavity sizes, presumably due to the quenched nematic morphology of the noncrystalline regions.