Abstract: The rate of carbon deposition and the carbon nanostructures formed during the chemical vapor deposition of ethane and synthetic natural gas, with and without added hydrogen, over a nickel catalyst, supported on threedimensional (3D) carbon fiber preforms, was investigated. Graphitic nanofibers, carbon nanotubes, and graphitic carbon shells were observed following carbon deposition; the nanostructured carbon deposited was dependant on deposition temperature and gas feedstock composition. Gas feedstocks ranging from volume fraction 100 to 20% hydrocarbon with a balance of hydrogen were investigated at temperatures ranging from 650 to 800℃, up to 12h total deposition time. The 3D carbon fiber supports were impregnated with a methanolic solution of nickel nitrate hexahydrate that was decomposed and reduced to nickel before carbon deposition. The samples were characterized using weight change measurements, thermogravimetric analysis coupled to a mass spectrometer, scanning and transmission electron microscopy, and X-ray diffraction. Analysis of the samples revealed an increase in the ratio of filamentous to encapsulation with increasing hydrogen content with the reverse being observed with increasing temperature. Unexpectedly, it was found that hydrogen addition did not extend the lifetime of the catalyst. This process shows good potential for the manufacture of carbon-carbon composites.