1. The conversions of cyclohexane, both pure and with additions of 0.5, 1.0, and 2.0% thiophene, were studied in the presence of 1% Pd-, Rh-, Ru-, and Pt-Al2O2 catalysts at 450°, hydrogen pressure 20 atm, ν=2 h−1, and molar ratio H2 ∶: HC =5∶:1. 2. Under the conditions of the dehydrogenation of pure cyclohexane, aluminopalladium and aluminorhodium catalysts possess lower dehydrogenating but higher isomerizing activities than an aluminoplatinum catalyst, while an aluminoruthenium catalyst is practically inactive under the conditions investigated. 3. In the presence of thiophene, the dehydrogenating activity of Pd- and Rh-Al2O3 catalysts decreases continuously; moreover, the depth of their poisoning increases with increasing poison concentration. The isomerizing ability of these catalysts, on the other hand, increases somewhat at the beginning of the experiments, then also gradually falls during the experiment. In the case of regeneration with pure cyclohexane, the dehydrogenating and isomerizing activities of the two catalysts are first restored, but then again begin to fall, passing through a maximum. 4. When the thiophene-S35 concentration in the initial cyclohexane is varied from 0.5 to 2.0%, the sulfur content in Pd-, Rh-, and Ru-Al2O3 catalysts increases; in all cases it is higher than in the aluminoplatinum catalyst. Sulfur is distributed uniformly along the layer of the inactive aluminoruthenium catalyst, while in the case of the Pd- and Rh-Al2O3 catalysts, the sulfur concentration gradually decreases from the upper portions of the layer to the lower portions along the path of the reaction mixture.