A complete study of the importance of the pyrolysis temperature (up to 1500 degrees C) of a petroleum residue (ethylene tar) in the activation with KOH of the resultant pyrolysis products (covering from the own ethylene tar to pitches and well developed cokes) has been carried out. The trend in the porosity found for activated carbons is as follows: the pore volume increases with the pyrolysis temperature reaching a maximum value (1.39 cm(3)/g) at about 460 degrees C, just at the transition temperature between a fluid pitch and a solid coke. It is the pitch with highest mesophase content that develops the maximum porosity when activated with KOH. The amount of H-2, CO and CO2 produced during the reaction of the mesophase pitch and coke with KOH has been quantified, and a trend as described for the pore volume was found with the pyrolysis temperature. Therefore, there is a relationship between the reactivity of the precursor with KOH and the porosity developed by the activated carbon. Since the reactions that produce H-2 initiate at temperatures as low as 300 degrees C, it seems that KOH is modifying the conditions under which the pyrolysis occurs, and this fact is critical in the development of porosity. (c) 2012 Elsevier B.V. All rights reserved.