Thermosetting polymers are widely used in electronic industry as encapsulants of electronic devices. It is well known that properties of polymers and polymer-based composites like molding compounds are highly dependent on conditions like: temperature, time, humidity, degree of cure etc. These effects are investigated extensively in the past. Surprisingly, the effect of postcure and thermal aging on the thermomechanical properties of molding compound is scarcely studied. Some studies are devoted to this topic but are not systematically carried out. The main conclusion of previous research is that the glass-transition temperature increases at increasing postcure time [1]. Also weight loss during aging is reported [2]. Since thermomechanical properties determine mainly the reliability of electronic devices it is essential to have knowledge of the effect of post mold cure treatment and thermal aging. In this research the influence of postcure and thermal aging is studied in a systematic way. It turns out that postcure and thermal aging treatment causes an increase in Tg, a change in viscoelastic behavior, an increase of the rubbery modulus, and ongoing shrinkage of the molding compound. The change of these properties is attributed to the oxidation of the molding compound at high temperatures.@en