Carbon‐13 nuclear magnetic resonance (NMR) has been used to investigate the structure and character of diglycidyl ether of bisphenol A (DGEBPA) epoxy resins more thoroughly than previously reported in the literature. It is shown that spectra, when run under quantitative conditions, can detect the loss of epoxides at the end groups by reactions such as hydrolysis. The products of acid hydrolysis of a DGEBPA resin are observed and assigned. It is further shown that the possibility of epoxide hydrolysis and impurities or structural irregularities can lead to inaccuracies in epoxy equivalent weight (EEW) calculations using a previously reported methodolgy. A new method is employed which can take into account loss of end group epoxides. Part of this new method consists of a measure of the epoxide to end group ratio (EER) which sheds light on the final possible quality of the cured resin. The EER value is shown to be in many ways more informative than the EEW measure which has the built‐in assumption that there is an epoxide group for each end group. Copyright © 1985 John Wiley & Sons, Inc.