Three-dimensional (3D) macro-porous structures composed by carbon nanotubes (CNTs) are synthesized by chemical vapor deposition (CVD) of methane at 1173K and atmospheric pressure on Co/Mo/Mg catalyst. 3D-structures are due to formation of interconnected CNT bundles. Strong metal/support interaction, breaking of the metal particles and large difference between surface- and bulk-diffusion are at base of the "double-base" growth mechanism, proposed to account for their formation. The variation of CH4 partial pressure (0.32-1.00 atm) reflects on changes in the products' morphology and carbon yield. These changes result to be determined by the competition between effects (release of H-2 and spreading and diffusion of the carbon atoms over and into the catalyst) promoted by the CH4 dissociation. At lower CH4 partial pressure the rate of dissolution of the C atoms is faster than that of the Co/Mo dispersion and bundled CNTs prevailingly form; at higher CH4 partial pressure the situation inverts and unbundled component of the 3D-structures increases. (C) 2015 Elsevier B.V. All rights reserved.