Plasma chemically modified carbon nanofibers were characterized by X-ray photoelectron spectroscopy with regard to the content of carbon, oxygen, and nitrogen and the contribution of carboxylic groups or ester, carbonyl and hydroxylic groups or ether on the surface. Unfortunately, X-ray photoelectron spectroscopy only provides an average value of the first 10 to 15 molecular layers. For comparison, depth profiles were measured and wet chemical methods were applied to estimate the thickness of the functionalized layer and the distribution of oxygen-containing functional groups within the near-surface layers. The results indicate that the fiber surface is covered by a monomolecular oxygen-containing layer and that plasma treatment allows a complete oxygen functionalization of the uppermost surface layer. The best conditions for plasma treatment found within the set of parameters applied to generate complete functionalization are: plasma gas O(2)/Ar ratio 1:1, gas pressure 1-1.5 hPa, plasma power 80 W, treatment time >or= 5 min. Additionally, three quick and easy methods are presented to estimate the efficiency of plasma treatment with regard to surface functionalization: pyrolysis, contact angle measurements, and light permeability measurements of aqueous carbon nanofiber suspensions.