The parallel dehydrogenation and dehydration of cyclohexanol and isopropanol on chromia show that the selectivity is determined, not by the catalyst alone, but by both, the absorbate and the catalyst. The dehydrogenation of the alcohols on chromia and chromia-alumina is enhanced when they are mixed with acetic acid or hydrogen. Cyclohexane suppresses the dehydrogenation. These observations are discussed in the light of the effect the chemisorption of the various compounds has on the electronic character of the catalyst. It is seen that the effect of doping semiconductor oxides can be simulated by mixing reactants with other substances. © 1968.