The porosity evolution in heavy oil fly ashes subjected to activation with CO2, has been examined. The work examined four different heavy oil fly ashes that, after preliminary acid leaching, have been pyrolyzed at 900 degreesC and then activated with CO2 at the same temperature for different times. A different evolution of porosity was observed according to the different reactivity of the samples during activation. The activated samples have been characterised as regards the surface area and the pore volume. The scanning electron microscope-energy dispersive spectrometer microanalysis has been used to interpret the experimental results. (C) 2003 Elsevier Science Ltd. All rights reserved.