The diamagnetic susceptibility of a series of random copolyesters, P(HB/HN), of 4-hydroxybenzoic acid (HBA) with 2-hydroxy-6-naphtoic acid (HNA) has been investigated, both, as a function of composition and temperature. It is shown that the molecular susceptibility of the uniaxially oriented samples linearly decreases with HE content. Results are discussed in terms of the increase in magnetic anisotropy occurring with the introduction of the HN groups. Furthermore, the temperature dependence of the specific susceptibility has been investigated. The diamagnetic susceptibility of these copolymers rises at the beginning of the solid-liquid crystalline transition. Above this temperature, the specific susceptibility exhibits a conspicuous time-dependent behaviour which is a function of the temperature at the mesophase. After long storage times at the mesophase, and upon cooling at room temperature, the susceptibility shows values which approach those of a macroscopically isotropic state, i.e., a state with no overall preferred orientation. X-ray diffraction measurements confirm a relaxation mechanism of the molecular orientation occurring at temperatures above the melting point. A similar relaxation process is observed for samples with an initial planar orientation.