While the electron microscope and adsorption isotherm techniques for evaluating the particle size and surface area of carbon blacks have been particularly valuable in interpreting their behavior in rubber, they also emphasize the fact that reinforcement cannot be interpreted solely on this basis. The activity or nature of the surface must also be considered. Heats of adsorption offer a means of evaluating surface activity. The differential heats of adsorption have been measured by precision calorimetry for nitrogen and a series of C4 hydrocarbons on a group of carbon blacks of varying reinforcing ability. Initial sites of high activity were detected. Surface activity decreases with increasing surface coverage. The magnitude of the initial heats parallels the reinforcing properties of the blacks studied. The surface activity and reinforcing ability of an MPC black was found to be greatly reduced by high temperature treatment. These effects were not influenced by the chemical nature of the surface, i.e., the presence of chemisorbed oxygen or volatile substances. In a further phase of this study, the nature of the surface oxide complexes present on carbon blacks has been studied by means of their emission band spectra in a special vacuum discharge tube. Aldehyde and carboxyl radicals were observed in high concentration. Hydroxyl radicals were also present in nearly equal concentration. These data offer a satisfactory interpretation of the pH properties of carbon black and also should prove of value in interpreting other chemical properties.