For epoxy adhesives based on the hardener dicyandiamide (Dicy), the literature offers different reaction schemes about how epoxy resin and Dicy react. Since these propositions are inconsistent, a complete description of the network formation regarding both tautomeric structures of Dicy does not exist currently. In this study, the oligomer formation before gelation is considered for a pre-cured epoxy system EP [diglycidylether of bisphenol A (DGEBA) + Dicy, mass ratio 100: 7] by reversed-phase HPLC, mass spectrometry, and FTIR spectroscopy analysis. The results verify that DGEBA and Dicy react to 2-cyanimido oxazolidine and ammonia or to 2-imino oxazolidine and cyanamide via Gilbert's or Zahir's cyclization, depending on the tautomer. Both chemical pathways go on simultaneously as proven for the first time. Other reactions proposed in the literature (transformation of imino ether to urea or urethane species) are not found under the given conditions (150 degrees C, 60 min).