A new family of cellular polymers-that, upon appropriate charging, reveals interesting piezoelectric properties-has been developed recently. Its major characteristic is thermal stability up to high temperatures, which broadens the potential field of application. These materials become electrets upon charging under high levels of electric field. Investigation of charging mechanisms is fundamental to reach optimization of electret and piezoelectric properties. This paper presents results and considerations relevant to contact charging of these new materials, pointing out the relation between partial discharge and space charge formation, as well as time and temperature stability of electret properties. The concepts of threshold for partial discharge inception and space charge accumulation, the modeling of avalanches in cavities, and the rate of charge generation are exploited trying to explain charging mechanism and, therefore, they provide useful indication for optimization of material manufacturing.