The mechanical and dc electrical properties of a carbon black-filled ethylene-octene elastomer (EO) are reported. The stress-strain curves of the composites, scaled with that of the unfilled polymer up to approximately 500% strain, suggest good filler-matrix adhesion. The large reinforcement effect of the filler followed the Guth model for non-spherical particles. Electrical behavior under large strain was divided in four qualitatively or quantitatively different regimes that differentiated carbon black composites with the EO matrix from carbon black composites with chemically vulcanized matrices. Among the most notable features of the EO composites was the completely reversible variation of the resistivity, up to 20% strain, suggesting that these materials might be useful as strain gauges. In addition, depending on the carbon black content, the composites retained low resistivity to high strains. From the results of various thermo-mechanical treatments, a microstructural model for the response to stretching was proposed. This model incorporated dynamic junctions specific to the EO matrix to describe the mechanical properties, the decrease in resistivity at low strains (20%), and the weak increase in resistivity at intermediate strains (up to about 300%).