We describe the impact of thermal treatment on the structure and phase transitions of the liquid crystalline aromatic co-polyester, HIQ-40, comprising 40 mol% p-hydroxybenzoic acid (H), 30 mol% isophthalic acid (1), and 30 mol% p-hydroquinone (Q). Simultaneous, real-time wide and small angle X-ray scattering (WAXS, SAXS), differential scanning calorimetry, and optical ellipsometry were used to study initially isotropic, amorphous films of HIQ-40. Films were annealed above the glass transition temperature, T-g at temperatures, T-a from 130 to 290 degreesC. Depending upon T-g thermal treatment results in formation of regions of nematic order and/or crystalline order in a disordered matrix. As T-a increases, molecular mobility in the amorphous phase increases resulting in a reduction in T-a Two or three endothermic events are seen in all samples by thermal analysis. The lowest temperature endotherm is associated with melting of crystals formed either at T-a or during the thermal scan. The two higher temperature endotherm features result from transformation of crystal melt-to-nematic, and formation of more mobile nematic domains from constrained liquid, respectively, and are relatively insensitive to T-a.A strong Bragg scattering peak is seen for T-u < 290 degreesC corresponding to formation of two-phase structure comprising crystals and disordered phase. At higher temperatures, very strong scattered intensity in the SAXS pattern re-emerges, even after all WAXS crystal reflections have disappeared. Results suggest that a two-phase structure, of ordered nematic domains co-existing with less ordered regions, may be forming continuously above the crystal melting point. (C) 2004 Elsevier Ltd. All rights reserved.