The growth of carbon nanofibers (CNF) was studied over a cordierite monolith previously coated with mordenite synthesized by a hydrothermal route. The zeolitic layer was deposited in order to have the sufficient number of cationic exchange sites to anchor the metallic particles responsible for the CNF formation. To produce the carbon nanofibers, cobalt was the active metal and acetylene, the source of carbon. The amount and type of nanocarbonaceous material obtained were studied as a function of the following operating conditions: (i) the reduction temperature of the metallic phase, (ii) the presence or absence of H-2 in the feed and (iii) the orientation of the monolith channels with respect to the flow direction. The results obtained indicate that under specific operating conditions, large amounts of fibers grow over the whole internal surface of the channels. These fibers provide a large specific surface, which indicates that this type of structured catalysts could be used as three-phase reactors. (c) 2007 Elsevier B.V. All rights reserved.