The interaction of 2-methoxypropene, a vinyl ether, with heterogeneous acid catalysts containing sulfonic acid groups covalently bound to SiO2 (Deloxan ASP, Degussa) and sulfuric acid adsorbed on TiO2-modified amorphous SiO2 (Degussa), respectively, was investigated by in situ attenuated total reflection infrared spectroscopy. Rapid hydrolysis is observed, which does not, however, require the acid sites. The resulting acetone is adsorbed predominantly on SiOH groups. Promoted by the acid sites a further transformation is observed on the catalysts. Based on the time behavior of the ATR signals of acetone and the product the further reaction likely involves the condensation of 2-methoxypropene and acetone. During the buildup of the reaction product hydronium ions disappear from the catalyst surface. Upon desorption of the reaction product the hydronium ions become prominent again on the catalyst containing adsorbed sulfuric acid. This behavior is less pronounced on the catalyst, which contains sulfonic acid groups. The two investigated catalysts contain vastly different relative concentrations of Bronsted and Lewis acid sites, which can explain the difference in the relative concentration of intermediate and product at the interface in the observed consecutive reaction. (C) 2003 Elsevier Inc. All rights reserved.