The effect of C-containing by products on the deactivation of Ni/La2O3-aAl2O3 catalyst in the steam reforming of ethanol (SRE) was studied. The catalyst was prepared by wet impregnation method and subsequently calcined at 550°C and reduced at 700°C, prior the reforming reactions. The kinetic runs were carried out in a fluidized bed reactor under the following operating conditions: atmospheric pressure; 500-1 °C; space-time, 5.2 and 9.2 gcatalyst min gEtOH and time on stream, 20 h. The properties of the fresh and used catalysts were analyzed by N2 adsorption-desorption, H2 chemisorption and X-ray diffraction (XRD), and the nature and amount of coke deposited on the catalyst were analyzed by Scanning Electron Microscopy (SEM) and Temperature Programmed Oxidation (TPO) analysis. It was found that the catalyst deactivation level is not directly dependent on the coke content but it highly depends on ethanol conversion: for a low conversion, the coke deposited encapsulates and blocks the metallic active sites, causing a fast deactivation of the catalyst, whereas for a high conversion the coke is deposited as filamentous carbon that contributes to decreasing the catalyst porosity but without affecting notably to catalyst stability. The different nature of the coke deposited on the catalyst is attributed to the intermediate compounds produced in the reaction medium, with acetaldehyde being the major responsible for the formation of encapsulating coke, whereas the CH4 and CO presence favors the formation of fibrous coke.