The effects of carbon black filler and polyethylene matrix morphology on void formation in conductive composites have been studied by small-angle neutron scattering. Void formation was observed to occur postprocessing during polyethylene crystallization as a result of filler particle/polymer matrix interactions. Carbon black structure, concentration and polymer crystallinity were identified as critical parameters in this process that can be combined and used to predict void content in these semicrystalline composite materials.