The stop-effect, a drastic increase of the reaction rate when the feed concn. of a reactant is switched to zero, was studied for the dehydration of ethanol to ethylene on y- alumina at 180 and 200 DegC. Two basic models exist in the literature to describe this phenomenon. They were discriminated on the basis of transient and periodic expts., coupled with FTIR data of the adsorbed species. The model that best describes these measurements postulates the adsorption of ethanol on two different sites, S1 and S2, with a free S2 site being necessary for ethylene formation. [on SciFinder (R)]