Two types of vapor-grown carbon nanofibers (CNFs), either as-produced or submitted to a graphitization treatment, were modified using microwave-generated oxygen plasma. Upon plasma treatment, a considerable amount of oxygenated species - mainly carboxylic acids, acid anhydrides and phenols - were introduced at the surfaces of both types of CNF, as found by X-ray photoemission spectroscopy (XPS) and temperature programmed desorption (TPD). Plasma treatment diminished the structural order, as established by Raman, while the porous texture, followed by nitrogen adsorption, barely changed. Plasma treatment is thus successful in introducing oxygen functionalities in both materials without significant modification of their textural properties, something very difficult to obtain with other treatments such as wet oxidation, most of all in the case of the graphitized CNFs. (C) 2013 Elsevier B.V. All rights reserved.