Carbonaceous materials (six carbon blacks) were characterized in terms of their surface area, porosity, pore-size distribution, and surface chemistry. Using p-nitrophenol in aqueous solution as the adsorbate, the calorimetric heat of adsorption was investigated in connection with the studied properties of the adsorbents. Techniques used were gas (N2, CO2) and p-nitrophenol adsorption, mercury porosimetry, density measurements, FT-IR, and microcalorimetry. The carbon blacks were included in two groups depending mainly on their porosity. The chemical nature of oxygen functional groups or structures was similar in all samples. Significant differences were observed in the concentration of surface oxygen groups. The heat of adsorption depended on the surface area and the microporosity of the samples. The density of oxygen groups was likely an important property in connection with the evolution of heat. The pore-size distribution of the adsorbents might also influence the heat of adsorption. When the amount adsorbed of p-nitrophenol and the heat of adsorption were expressed on a per unit surface area basis, good agreement with the FT-IR results was noted.