A new procedure to obtain high molecular weight poly(1,2-dihydroxybenzene phthalates) by bulk polymerization of some spiro-compounds is described. O,O’-Phthalid-3-ylidenecatechol (I) and 4-methyl-O,O’-phthalid-3-ylidenecatechol (II) spiro-compounds have been synthesized and used as monomers. Poly(1,2-dihydroxybenzene phthalate) (III) and poly(1,2-dihydroxy-4-methylbenzene phthalate) (IV) have been prepared by heating at 200 degrees C of spiro-compounds I and II, respectively, without addition of catalyst. The resulting polyesters III and TV have been characterized by GPC, TG, DSC, NMR, FAB-MS, and MALDI-TOF MS analyses. The GPC curves of the obtained materials showed that high molecular weight polyesters, containing only a very little amount of low molecular weight oligomers, have been formed. The structural characterization of these polyesters by FAB and MALDI-TOF mass spectrometric analyses showed that they are constituted exclusively of open-chain macromolecules having both hydroxy end-groups. Data collected suggest that in the bulk polymerization of spiro-compounds I and II a possible chain growth mechanism of polymerization is active, which is strongly inhibited by acid species.