Multi-walled carbon nanotubes (MWCNTs) have been produced with high selectivity by fluidized bed catalytic Chemical Vapor Deposition from ethylene on Fe/Al2O3 catalysts. The influence of operating parameters such as deposition duration, temperature, ethylene and hydrogen partial pressures and iron loading, on MWCNT productivity, process selectivity, characteristics of final powders and chemical composition of the outlet gases has been analyzed. Using gas phase chromatography, methane and ethane have been detected whatever the conditions used. Between 650 and 750°C, no catalyst deactivation occurs because nucleation remains active all along the synthesis thanks to the explosion of the catalyst grains. Above 650°C, ethane itself produces MWCNTs, whereas methane does not react in the temperature range 550-750°C. The formation of MWCNTs induces marked bed expansions and sharp decreases of grain density. Apparent kinetic laws have been deduced from the collected data. The apparent partial orders of reaction for ethylene, hydrogen and iron were found to be 0.75, 0 and 0.28 respectively.