The effect of liquid-phase oxidation on the texture and surface properties of carbon nanofibers has been studied using XRD, TEM, SEM, N 2-physisorption, TGA-MS, XPS and acid-base titrations. Oxidation was performed by refluxing the nanofibers in HNO 3 and mixtures of HNO 3 /H 2 SO 4 for different times. The graphite-like structure of the treated fibers remained intact, however, the specific surface area and the pore volume increased with the severity of oxidation treatment. For the first time it is shown that the most predominant effect that gives rise to these textural modifications is the opening of the inner tubes of the fibers. Moreover, it is demonstrated that both the total oxygen content (O/C ¼ 0.02-0.07 at/at) as well as the number of acidic groups (1-3 nm À2) are a function of the type of oxidizing agent used and the treatment time. The total oxygen content of the oxidized samples turns out to be substantially higher than can be accommodated in the form of oxygen-containing groups at the exterior surface.