A new epoxy/episulfide is investigated using a dicyandiamide (DICY) curing agent system. The resin exhibits better adhesion to copper, a lower thermal expansion coefficient, a lower heat of reaction, and slightly higher water absorption as compared with the standard epoxy system. The dielectric constant of the epoxy/episulfide system is almost the same as that of the standard epoxy system. The properties are due to the reaction mechanism change caused by the addition of episulfide, which is studied using a monofunctional model compound system; the amino groups in DICY initially react much more easily with the episulfide than with the epoxy ring. The S- formed by this reaction reacts with the episulfide and the epoxy quickly. In the presence of copper, the episulfide and/or the S- also react with copper, forming a durable bond between the copper and matrix resin that retains strength even after water boiling.