Solid epoxy resins are produced at Vantico SA by catalyzed reactions of a low-mol. wt. liq. resin (RBL) with bisphenol A (BPA). These reactions lead to solid epoxy resins, which are used in surface coatings or adhesives. The chain lengthening step (advancement reaction) is based on the reaction of the epoxy functional group with the phenolic hydroxyl group leading to the formation of a secondary hydroxyl group. These secondary hydroxyl groups can also react with epoxy groups to form branched mols. The authors present a kinetic model describing the transformation of BPA with a RBL up to complete conversion of BPA. The polymn. kinetics is investigated by isothermal measurements for temps. between 180 and 210 Deg and for different initial concns. of BPA. A reaction scheme describing both the advancement and the branching reactions was developed. For modeling the branching rate, a detailed diffusional and semiempirical model was adapted and modified in order to fit the exptl. results.