Degradation of wire bonds (WBs) is one of the major factors limiting reliability of plastic encapsulated microcircuits (PEMs) at high temperatures. Use of PEMs in military and aerospace applications requires extended and thorough evaluation of encapsulating materials and reliability of packages in harsh environments. However, the effect of environmental conditions on characteristics of molding compounds (MCs) and reliability of wire bonds has not been studied sufficiently to date. In this work, two types of PEMs in QFP-style packages have been stored in different environments at temperatures from 130 ^{oC to 225 oC for up to 4,500 hours in some cases. To assess the effect of oxygen, the parts were aged at 198 oC in air and vacuum chambers. The effect of humidity was evaluated during long-term highly accelerated temperature and humidity stress testing (HAST) at temperatures of 130 oC and 150 oC.}