Electron paramagnetic resonance (EPR) has now become firmly established as one of the methods of choice for analyzing the carbon network over a range of different volume fraction of the carbon black in the composite, i.e., below and above the respective conduction threshold concentration. In the present article, two types of carbon blacks, having very different primary structures, surface areas, and percolation thresholds, were used; Raven 7000 (of high surface area and high percolation threshold volume fraction) and Y50A (of low surface area and low percolation threshold volume fraction). A semiquantitative image analysis of the microstructure from transmission electron microscopy reveals information about the spatial distribution of the carbon aggregates and agglomerates inside the composite. We observe that the apparent surface of agglomerates increases significantly with increasing carbon black content for the two types of blacks investigated. Adsorbed oxygen on the carbon black cristallites and dynamic coalescence under mixing conditions can be responsible for the broadening of the dispersed phase surface distribution. The interagglomerate distance in two samples of concentrations f≪fc and f≅fc of Raven 7000 are nearly identical indicating that the dc condition threshold can therefore be almost entirely attributed to the coalescence of smaller aggregates. Line shape simulation showed that the changes in the absorption EPR spectra, at temperatures between 105 and 300 K, of the composite samples containing Raven 7000 can be described by a linear superposition of two distinct Lorentzian (one broad and the other narrow) resonance lines and a single (narrow) Lorentzian resonance line for composite samples containing Y50A. The spins giving rise to the EPR signal reside in the carbon black particles. In Raven 7000, the significant difference in linewidth between the two signals demonstrates a - different environment where the restriction of the motion of the paramagnetic centers varies. The narrower line was assigned to spin probes with high mobility (carbon black aggregates) and the broad one to probes with restricted mobility incorporated in carbon black agglomerates. In Y50A, only the sites with high mobility were detected. When the temperature is increased the data demonstrate that the EPR signal intensity, which is the double integral in arbitrary units divided by the mass of the carbon black contained in the sample, decreases slowly in the temperature range 105–300 K. The various phenomena observed are attributed mainly to the aggregates and agglomerates structure in the composite samples. The temperature dependence of the paramagnetic susceptibility deduced from the EPR integrated intensity is discussed in terms of Adriaanse etal’s model [L. J. Adriaanse, J. A. Reedijk, P. A. A. Teunissen, H. B. Brom, M. A. J. Michels, and J. C. M. Brokken-Zijp, Phys. Rev. Lett. 78, 1755 (1997)]. The magnetic susceptibility of the composite samples is also measured with a superconducting quantum interference device magnetometer, operating at an applied magnetic field of 0.5 T, from 2 K to room temperature. The observed temperature dependence of the spin susceptibility is discussed and suggests that morphology heterogeneity is of overwhelming importance to understand the magnetic properties of these materials. © 2001 American Institute of Physics.