In situ coupled mechanical/electrical/WAXS/SAXS investigations on EPDM-based composite materials filled with carbon black (CB) have been performed under uniaxial tensile stretching. The time-resolved correlation between the electrical conductivity and the microstructural state of the composite reveals the mechanisms governing the structure/properties relationships with increasing deformation: orientation of the EPDM chains with reorganization of the CB network and ultimately, nano-cavitation. This involvement of different microstructural phenomena offers a comprehensive picture of the mechanisms underlying the non-monotonic evolution of the electrical conductivity with strain. At large deformations, the formation of nanovoids preceding the material fracture induces by a significant decrease in conductivity. Our study thus brings new light on the conductivity/strain relationships of carbon black filled elastomers. We envision potential applications in developing smart rubber materials where microstructural changes preceding material rupture can be monitored in situ by coupled electrical measurements.