Various oligo- and poly(ethylene glycol)s were transformed into tin-containing macrocycles by "polycondensation" with dibutyltin dimethoxide. These cyclic oligo- and poly(ethylene glycol)s were heated with a spirocycle derived from phthaloyl chloride and 1,2-dimercaptoethane. This spirocycle inserts into the Sn-O bonds with subsequent elimination of 2,2-dibutyl-2-stanna-1,3-dithiolane. By this ring-insertion/ring elimination mechanism, the Bu2Sn group in the cyclic polyethers was replaced by the phthalate unit without intermediate ring-opening. The cyclic poly(ethylene glycol) phthalates were characterized by elemental analyses by viscosity and DSC measurements, by H-1 NMR, C-13 NMR, and IR spectroscopy, and by MALDI-TOF mass spectrometry. Analogous experiments were conducted with a spirocycle derived from 4,5-dichlorophthalic acid which turned out to be less reactive than the unsubstituted phthalide. In summary, this new approach allows syntheses of cyclic polyether phthalates in high yields, in an "one-pot procedure" and without need of dilution.