Effect of carbon black (CB) on the morphology of filled polyethylene (PE)/polystyrene (PS) blends has been investigated by image analysis of optical micrographs (2-D analysis) and by the selective extraction of one phase of the binary blends (3-D analysis). The macroscopic electrical resistivity of the filled polyblends strongly depends on the selective localization of CB in one phase or at the interface and above all on the double percolation, i.e. percolation of the polymer phases and percolation of the CB particles. The selective localization of CB in the PE phase has remarkable effects on the polyblend phase morphology. The phase cocontinuity is indeed extended over a much larger composition range, and the phase morphology is stabilized toward post-thermal treatment at 200 °C. In the case of double percolation and selective localization of CB at the polyblend interface, electrical conductivity is observed at a CB content as low as 0.4 wt %.