Aromatic polyesters containing peroxide groups incorporated in the polymer chains were synthesized by interfacial polycon-densation of bisphenol A, sodium peroxide, and isophthaloyl chloride. Iodometric titration, elemental analysis, and IR spectra showed that the polymers obtained possessed a high peroxide content, even though bisphenol A reacted 80 times faster than Na2O2with isophthaloyl chloride. Analysis of the polymers by high resolution NMR and degradation of the chains with subsequent gel permeation chromatography measurements showed that the distribution of peroxide groups is random in the case of low peroxide content. Higher peroxide content leads to the formation of peroxide blocks in the polymer as well as greater polydispersity of the polyester fragments formed after degradation of the peroxide links.