We investigated the influence of sulfur on a well-known methanation catalyst, nickel on alumina/silica, so as to gain more information about sulfur poisoning. By different measurement techniques (DRIFTS and IR spectroscopy) we were able to show that the higher the sulfur content in the sample, the lower the amount of adsorbates on the surface of the catalyst. Nickel sulfide was completely inactive for the CO2 reduction. Interestingly, we found active surfaces containing sulfur, such as molybdenum sulfide, which catalyzes the reverse water gas shift reaction (conversion of CO2 to CO). Therefore, we know that sulfur influences the methanation of CO2, however, still allows the transformation of CO2 to CO. We were able to gain a broader understanding about the influence of sulfur and the mechanism, however, other techniques are required in order to identify hydrogen containing intermediates.