The microhardness of a series of random copolyesters of 4-hydroxybenzoic (HBA) and 2-hydroxy-6-naphthoic acid (HNA) has been investigated as a function of composition and temperature. The results reveal that, at room temperature, the microhardness of non-oriented materials deviates from the linear additivity of the hardness of single homopolymers. Such a deviation is shown to be mainly related to changes in the molecular packing of the rigid chains. This packing, and as a consequence microhardness, can be characterized by an average cross-sectional area which includes crystalline and non-crystalline regions. The fact that the microhardness behaviour and the temperature dependence of the cross-sectional area are closely related is emphasized. An analytical expression embracing the coefficient of thermal softening, beta, and the thermal expansion coefficient of the cross-sectional area, alpha(A), is proposed.