Two activated carbons were prepared from olive stones with different degrees of activation and oxidized with a (NH4)(2)S2O8 solution for variable periods of time to introduce different amounts of oxygen surface complexes. Samples so prepared were characterized to know their surface area, porosity, and surface chemistry and then were used as catalysts in the conversion reactions of ethanol. The dehydration reactions to obtain ethene and ether only occurred on the oxidized samples, where carboxyl groups placed on the external surface of the particles were responsible for these reactions. The dehydrogenation reaction to yield acetaldehyde took place on either acid or basic surface sites placed not only on the external surface, but also on some part of the internal surface. During the reaction some surface active sites for dehydrogenation were lost because some hydrogen remained bound to them. The presence of air in the reactant mixture cleaned some of these sites, increasing the dehydrogenation activity and decreasing the dehydration activity. Also studied was the effect on their activity of the pretreatments of the catalysts in He or H-2 flow at different temperatures up to 1273 K.