The thermal degradation processes occurring in the pyrolysis of the isomeric aromatic polyesters I-III synthesized by condensation reaction between phthaloyl chloride and catechol, resorcinol, or hydroquinone, respectively, were investigated. Data obtained by direct pyrolysis mass spectrometry (DP-MS) indicate that the thermal decomposition processes of these polymers yield only and selectively cyclic 1,2-dihydroxybenzene phthalate, bis(1,3-dihydroxybenzene phthalate), and tris(1,4-dihydroxybenzene phthalate) as a function of the polymeric structure. These results were also confirmed by GPC and MS analyses of the pyrolytic materials collected during the thermal degradation experiments under vacuum of polymers I-III. As a consequence, it is possible to identify each of the three isomeric polymers just on the basis of their DP-MS spectra.