The interactions between hydrogen-rich plasmas and nondiamond phases contained in nanocrystalline diamond films have been examined to explore a possibility of reducing the amount of nondiamond phases. Diamond films with average grain sizes of 15-25 nm were deposited on silicon in microwave Ar-rich/H-2/CH4 plasmas. The 1140 cm(-1) peak in the visible Raman spectra for the films was used as a signature of trans-polyacetylene (TPA) segments. The amount of TPA relative to amorphous carbon increased with increasing H-2/CH4 ratio in the Ar-rich/H-2/CH4 plasmas or simply by exposing the as-grown film to hydrogen plasmas after deposition. However, the Raman peak of diamond, which was superimposed by the D peak of amorphous carbon, was little enhanced by exposure to these hydrogen-rich plasmas. It was also found that an increase in the relative amount of TPA was not always accompanied by an increase in hydrogen content in the films. The results were interpreted along the possible mechanisms of TPA formation, which are correlated to the content of hydrogen in the gas phase as well as in the solid phase. (c) 2006 American Vacuum Society.