Several models have been suggested to interpret the positive effect of Bi, Pb, and other metal promoters in the oxidative dehydrogenation of alcohols over Pt-group metal catalysts, though the experimental proof to these models is mainly missing. Here we propose a new and simple approach for clarifying the promoter effect: the comparison of dehydrogenation in inert atmosphere and oxidative dehydrogenation in the presence of oxygen over promoted and unpromoted catalysts. As test reactions the transformations of 1-phenylethanol, 2-octanol, and cinnamyl alcohol to the corresponding carbonyl compounds in a slurry reactor are used. The observed promoter effects can be classified into three groups: (i) acceleration of the hydrogen abstraction (dehydrogenation) step, (ii) acceleration of a reaction involving the transfer of (chemisorbed) oxygen, e.g., oxidation of the coproduct hydrogen or a surface impurity, thus generating free active sites, and (iii) a combination of these two different processes. This discrimination revealed that the metal promoter may have a dual effect on the aerobic oxidation of alcohols, and this behavior may rationalize some former controversial interpretations. Besides, this study presents the first evidence for the promoting effect of Bi in alcohol dehydrogenation in the absence of oxygen. (C) 2004 Elsevier Inc. All rights reserved.