The changes produced in an activated carbon by treatment in aqueous solutions of (NH4)(2)S2O8 and H2O2 are compared with the modifications produced by oxygen plasma. These changes are monitored by measuring the textural characteristics (surface areas, porosity, pore size distributions) and surface chemical groups. The former are studied by N-2 and CO2 adsorption, mercury porosimetry and hydrocarbon adsorption at zero surface coverage. The chemical surface groups are determined by chemical titrations and FTIR Spectroscopy. The results show large differences between the changes produced by treatment in aqueous solutions and those produced by oxygen plasma. Whereas both treatments in aqueous solution modify the textural characteristics, oxygen plasma maintains the surface area and pore network. However, plasma treatment introduces larger amounts of oxygen chemical groups than treatment in aqueous solution.