The effect of pre-oxidation on the porosity evolution in heavy-oil fly ash subjected to activation with CO, has been investigated. After preliminary acid leaching, used to reduce the mineral matter content, the leached fly ash has been oxidised in air at 250 degrees C for 36 h. Pyrolysis was conducted on the unoxidised and oxidised leached fly ash at 900 degrees C for 2 h and the resultant chars were activated with CO2 at 900 degrees C for different times. The activated samples have been characterised as regards the surface area and the pore volume. The pre-oxidation enhances the porosity development mainly in terms of mesoporosity leading to obtain activated products with higher surface area (about 270 m(2)/g at a 40% burn-off). (c) 2005 Elsevier Ltd. All rights reserved.