This work studies the mechanism of coke deactivation of a Ni/La2O3–αAl2O3 catalyst in ethanol steam reforming conducted in a fluidized bed reactor under conditions of severe deactivation, at 500 °C. Deactivation takes place in three consecutive stages with time on stream, corresponding to increasing values of ethanol concentration in the reaction system and decreasing values for CH4 and CO byproducts. The analysis of the nature of coke by means of several techniques in the different deactivation stages shows that high conversion values (low ethanol concentrations in the medium) give way to filamentous coke (with CO and CH4 as precursor), which has little effect on deactivation. For a highly deactivated catalyst, ethanol concentration is high, and a nonfilamentous deactivating coke is formed (mainly due to the evolution of filamentous coke). X-ray diffraction analysis of Ni particles in different deactivation states shows that a fraction of Ni crystallites are dragged by filamentous coke and the fraction of Ni crystallites that remain supported are blocked by nonfilamentous coke. A scheme has been established to explain the evolution of Ni and coke, the interaction between both dynamics, and its effect on catalyst deactivation.