An electronic device cannot perform its designed functions until it is packaged such that it is interconnected with the rest of the system and protected. As an encapsulation material, thermosetting polymers are widely used. It is well known that properties of polymerbased composites like molding compounds are highly affected by the influence of temperature, relative humidity and degree of conversion. The effect of above mentioned internal and external circumstances are investigated extensively in the past. Surprisingly the effect of high temperature storage on the mechanical properties is scarcely studied. From literatures research it is concluded that high temperature storage and postcure treatments increases the glass transition temperature. Also a weight loss during high temperature storage is reported [1], [2]. Since thermal treatments are very common in micro electronic industry, it is relevant to understand in more detail the influence of thermal aging on the mechanical properties of molding compounds. In this research the effect of postcure and thermal aging on the mechanical properties of molding compounds is systematically studied. It turns out that postcure and thermal aging cause a broadening of the viscoelastic region, an increase in Tg, and increase in rubbery modulus and ongoing shrinkage of the molding compound. It is concluded that the change in properties can be attributed to a reaction of the molding compound with oxygen. This paper presents the observed change in properties caused by oxidation.@en