The preparation of the addition products of H3PO4 to bisphenol A based epoxy resins is described. These products are considered modifiers of the epoxy resins and for the further formation of useful networks (coatings and related products). Depending on the ratio of H3PO4 to epoxy groups and other reaction variables, a family of oligomers with acidic end groups has been formed. The major reaction is the addition of H3PO4 to epoxy groups. Among the side reactions, the acid-catalyzed addition of the >CH-OH (and/or -CH2OH) groups to epoxy groups dominates. The -OH groups, if not present in the starting resins, are emerging from the P-OH to oxirane ring addition. The matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of the starting epoxy resins has revealed their structural diversity. In the kinetic measurements and determination of the ester composition, diglycidyl ether of bisphenol A (BADE) as a model compound has been mostly applied. The kinetics of the H3PO4-BADE reaction is described, and the structures of the final products are analyzed with P-31{H-1}-NMR, which gives (at pH similar to 12) quantitatively the proportions of the monoesters, diesters, and triesters as well as the unreacted H3PO4. The conditions have been found leading exclusively to the monoester, and this indicates that an increase in the reaction temperature enhances the selectivity of the reaction; that is, it depresses the proportion of the acid-catalyzed -OH and epoxy group reaction. (C) 2007 Wiley Periodicals, Inc.