Combined application of kinetic measurements, SSITKA and deuterium tracing techniques allowed to elucidate the mechanism of the key steps of butadiene synthesis over silver promoted zirconia catalysts, including ethanol dehydrogenation, acetaldehyde aldol condensation, and crotonaldehyde reduction with ethanol, and to determine the rate-limiting step of the process. We show for the first time that butadiene synthesis involves two independent catalytic cycles: i) dehydrogenation of ethanol into acetaldehyde over metal sites, and ii) acetaldehyde/ethanol transformation into butadiene over Lewis acidic sites. The first cycle implies ethanol dehydrogenation into acetaldehyde via concerted cleavage of CH2 and OH groups, followed by the fast desorption of the products formed into the gaseous phase. The second cycle starts with the activation of acetaldehyde over Lewis acid sites through enolization and its interaction with another acetaldehyde molecule from the gaseous phase via Eley-Rideal mechanism. The formed 3hydroxybutanal further dehydrates into crotonaldehyde. The aldol condensation step was proposed to be rate-determining. Further transformation of crotonaldehyde proceeds through Langmuir-Hinshelwood mechanism, involving the interaction of crotonaldehyde with ethanol over a Lewis site via a six-member ring transition state. Subsequently, crotyl alcohol dehydrates into butadiene, and acetaldehyde adsorbed over Lewis sites initiates the next catalytic cycle. The proposed molecular-level mechanism gives insights into rational design of efficient catalysts for the ethanol conversion into butadiene. (C) 2017 Elsevier B.V. All rights reserved.