Abstract: An epoxy resin encapsulating composition for semiconductor packages consisting essentially of an epoxy resin adduct of epichlorohydrin and an aromatic hydroxyl-containing compound, and an inorganic ion-exchange solid material admixed with the epoxy resin adduct. The ion-exchange solid material is selected to have the capability of exchanging its mobile anions with the undesired chlorine ions which occur inherently in the package made from the encapsulating composition and are very likely to corrode the metallic conductor on the semiconductor to be encapsulated. The ion-exchange solid material is characterized to include antimony and bismuth as basic materials forming an ion-exchange network which carries the mobile anions to be substituted with the chlorine ions. This ion-exchange material also is found to be also effective for trapping undesired alkali metal ions, for example, Na.sup.