The results of electrical conductivity and electron spin resonance (ESR) measurements on NR with four types of carbon black filler were reported. Two forms of space change limited current (SCLC) were identified, indicative of different electron trap energies, depending on the type of carbon black filler. Such SCLC behavior, with steep trap-filling regions on the current-voltage curves, is indicative of trap-dominated transport in the polymer regions of carbon-black—NR composites. Discrete shallow traps are evident for two of the compositions; deeper, exponentially distributed traps for two others. ESR spectra of the composites are very pronounced, with lineshape also depending on the type of carbon black used. Observed g-values are close to that of the free electron, and don't vary with carbon black type or temperature. Linewidths narrow and increase in intensity with increasing amount of carbon black, with no detectable g-value shift, indicative of exchange narrowing from nonorbital electrons. The SCLC and ESR results are consistent with a model of electron-hopping transport, with traps playing a prominent role, the trap distribution depending on type of carbon black in the matrix.