A specially constructed all-glass reaction system was used to reinvestigate hydrogen spillover through the gas phase with extremely purified hydrogen, using alkene hydrogenations in the gas phase as test reactions. The spillover reaction could he observed even with hydrogen - with less than 0.01 ppm O-2 - which additionally was passed over the same type of Pt/Al2O3, catalyst at the beginning of the line, in a second group of experiments the effect of oxygen traces was investigated, which were added to the hydrogen stream. Small oxygen contents (less than or equal to 50 ppm) did not influence the reaction, higher contents (greater than or equal to 100 ppm) caused reduced conversion.