A simple model is proposed assuming location of the CB in the PS phase only and continuity of the PS phase. The figure shows the percolation threshold ϕc of PS/PMMA/CB composites as a function of PMMA content. Insert: Schematic illustration of the volume for PS/PMMA/CB system. Display omitted •CB preferentially locates to the PS phase in immiscible PS/PMMA blends.•PS/PMMA (30/70) blends showed the lowest electrical percolation threshold.•A simple model is suggested to describe the percolation behavior.•After annealing, the CB affected the stabilization of the co-continuous structure. Blends of carbon black (CB)-filled polystyrene (PS) and poly(methyl methacrylate) (PMMA) with different PS/PMMA ratios have been prepared by melt blending in order to obtain conductive polymer composites with a low electrical percolation threshold according to the concept of double percolation. The dependence of the electrical conductivity on the morphology of the composite was examined before and after annealing in the molten state. The electrical conductivity of the composites was highest when the PS and PMMA phases formed a co-continuous structure and CB particles were selectively located and percolated in the PS phase. PS/PMMA (30/70) blends exhibited the lowest electrical percolation threshold of 1.26vol% CB. A simple model is suggested to explain this effect. After annealing, the CB clearly affected the stabilization of the co-continuous structure. Moreover, the electrical conductivity increased after annealing and the percolation threshold decreased to below 1vol% CB regardless of PS content. The largest increase in electrical conductivity after annealing was found with PS/PMMA (70/30) blends filled with 1vol% CB. With increasing PMMA content, this effect became less pronounced. The different behaviors in the composites can be explained by dynamic percolation and decrease in continuity of the CB-filled PS phase induced by structure coarsening.