Multiwall carbon nanotubes (MWNTs) were grown by catalytic chemical vapor deposition (CCVD) with ethylene as the carbon feedstock gas. Gibbsite was used as the support for Co and Fe. Different synthesis operating conditions were investigated with the aim of obtaining a high yield of carbon nanotubes. Moreover, the reactor outlet gas was continuously monitored on-line also during the catalyst activation and deactivation steps. Several techniques were utilized to characterize the catalyst and reaction products and to correlate their properties with the growth mechanism and operating conditions. The stability of the catalyst activity was also investigated. The carbon content in the synthesis product increased as the temperature, ratio of feed flow/catalyst mass, C2H4 partial pressure and reaction time increased. With increasing temperature, an increase in ethylene conversion was observed, catalyst efficiency and lifetime ensure very high yields and selectivity. The catalytic effect of Al2O3 is established, a growth mechanism in particular in terms of the support role, is proposed. Basing essentially on the microscopy characterization, it was also found that not only size but also chemical composition influences the nanoparticles catalyst's activity. (C) 2012 Elsevier B.V. All rights reserved.