LC and HPLC chromatographic experiments were made to separate the low molecular weight oligomers contained in three totally aromatic polyesters obtained by the condensation reaction between phthaloyl dichloride and catechol, 4-methylcatechol, or dithiocatechol. The characterization of the extracted oligomers was performed by techniques of thermal analysis, i.e. differential scanning calorimetry (DSC) and thermogravimetry (TG), and techniques of structural identification, i.e. nuclear magnetic resonance (NMR), direct pyrolysis mass spectrometry (DP-MS), fast-atom bombardment mass spectrometry (FAB-MS), and X-ray diffraction analysis. Mass spectrometric data show that the lower molecular weight product contained in each of the three polymeric materials has the following composition : C14H8O4 (mw = 240), C15H10O4 (mw = 254) or C14H8O2S2 (mw = 272). All the data collected in this work, and in particular those obtained by NMR and X-ray diffraction analyses, show that these products have a spiro structure, whereas the other extracted products with higher molecular weight are, as expected, cyclic and linear oligomers having ester linkages.