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An appreciable concentration of unpaired electrons has been found in colloidal carbon blacks through the application of electron spin resonance spectroscopy. The unpaired electrons in a typical channel process carbon black, Spheron 6, have been studied by noting the changes in the electron spin resonance spectrum produced by heat treatment (HT) to 3000°C and by various degrees of evacuation of the sample. The results indicate that the unpaired electrons in the original material are mobile π electrons, which are stabilized by condensed aromatic ring structures. Their concentration decreases slowly to 750°C HT and then declines more rapidly and vanishes above 1400°C HT. A different species of unpaired electrons arises at 1400°C HT. These are believed to be immobile σ electrons resulting from broken carbon‐carbon bonds at lattice defects. The unpaired electrons present in several commercial carbon blacks have been examined and are believed to be π electrons also. A tentative theory of the origin of the unpaired electrons is given.