Electrical resistivity and morphology of high impact polystyrene (HIPS)/styrene-isoprene-styrene copolymer (SIS)/carbon black (CB) blends were studied, Conductive CB particles locate preferentially within the MPS phase of the HIPS/SIS blends, The blends studied remain conductive as long as HIPS maintains a continuous phase and the effective CB concentration within HIPS surmounts its percolation threshold, Thus, blends containing 2 phr CB depict significant changes in resistivity with the HIPS/SIS composition, transforming from insulative to conductive. SIS/CB mixtures exhibit an unusual behavior, explained by a physical model suggested in this paper an extended to the HIPS/SIS/CB systems.