Several series of activated carbons were prepared from olive stones by means of carbonization followed by activation with carbon dioxide, water steam and a mixture of them, under different experimental conditions. The changes in porosity of the original char during activation were studied by adsorption of N-2 at 77 K, CO2 at 273 K and Hg porosimetry. The study was carried out covering a wide range of burn-off (19-83%) using activation times of 20-120 min, and temperatures between 650 and 950 degrees C. It is shown quantitatively how the individual factors influence the development of microporosity. It was found that in general terms, increasing activation produces a continuous increase in the volume of micropores and mesopores. However, this development occurs in a different proportion whether CO2 or steam are used: while CO2 produces narrow micropores on the carbons and widens them as time is increased, steam yields pores of all the sizes from the early stages of the process. The simultaneous use of these two activating agents resulted positive at times higher than 1 h, since it yielded carbons with higher volumes of pores. (C) 2008 Elsevier B.V. All rights reserved.