The plasma treatment and characterization of carbon nanofibers (CNFs) using vapors of ammonia/water (NH3/H2O) is reported. CNFs were treated at an input power of 20 and 40 W for 60 min using an inductive RF plasma of 13.56 MHz. During the plasma treatment, the ions were measured in situ using an optical emission spectrometer. Changes in the chemical composition, morphology and percent of functional groups grafted onto the CNFs were studied by infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetrical analysis. As a result of the treatment, a higher oxygen and nitrogen concentration on the surface of CNFs has been obtained, resulting in a good dispersion of the treated CNFs in water and acetone. The emission spectra indicated the formation of ions of CO, which are associated with the purification of the CNFs as indicated by an increase of the sp(2)/sp(3) ratio from 4.3 to 7.5. The fibrillar structure of the treated CNFs was not damaged by the NH3/H2O plasma treatment.