Mixtures of DGEBA with gamma-valerolactone (gamma-VL) or alpha-methyl-gamma-butyrolactone (gamma-MBL) 2:1 (mol/mol) were cationically copolymerized in the presence of scandium, ytterbium, or lanthanum triflates as initiators. The evolution of the different reactive groups was followed by means of FTIR/ATR spectroscopy. From these experiments, we could detect the coexistence of two unexpected processes: a reversion of the intermediate spiroorthoester formed to the initial products and a depolymerization process, which only takes place in samples with gamma-VL, when scandium triflate was used as initiator or when the proportion of ytterbium triflate was increased from 1 to 3 phr. When gamma-MBL was used as comonomer no depolymerization occurs which supports the proposed mechanism. (C) 2007 Wiley Periodicals, Inc.