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Although dynamic mechanical properties of thermoplastic polymers have been studied in great detail and much has been learned about their structure, little has been done in the field of thermosetting resins. The present study involves the use of dynamic methods for determining certain salient features about epoxy resins. A torsion pendulum operating in the range of one cycle per second was used to measure the properties of various epoxy systems. The instrument enabled rapid determination of both internal friction and shear moduli. Two transition peaks in the dynamic loss tangent were observed; one occurring at −60°C, the other at 165°C when a conventional epoxy resin was cured with m‐phenylenediamine. The low temperature peak was effectively demonstrated as due to the glycidyl ether portion of the cured resin system. The major peak or glass transition temperature could be varied in accord with the substituents on the central carbon bridge of the bisphenol portion of the molecule and to a lesser extent by the degree of cross‐linking.