To study the function of the carbon network, electrical conductivities and dielectric constants were measured on carbon black filled vulcanizates through the full range of elongation. Data were obtained for a variety of elastomers and carbon blacks, at different loading levels and covering the commercial particle size and structure ranges. The influence of the temperature and strain rate was investigated. In most of the reinforcing blacks there is a pronounced fall in conductivity and a minor decrease in dielectric constant in the first area of elongation, corresponding to the elimination of the transient structure. This is followed by an area of steady increase, caused by alignment of persistent carbon chains. At still higher elongations a sharp decrease occurs, due to destruction of carbon-elastomer bonds. The first two effects are very pronounced in high structure blacks but negligible in very low structure reinforcing blacks. The third effect leads to hysteresis and irreversible changes in the vulcanizates. Graphitized carbon black, with little or no bonding to the elastomer, does not show the third effect. Particle-elastomer adhesion data, obtained by an electron micrographic replica technique, independently confirmed the results. Anisotropy in electrical conductivity was observed. The major significance of the carbon filler in stress softening is discussed.