Simple test reactions as ethene hydrogenation, 2-butene cis-trans isomerization and H-2/D-2 scrambling were shown to be catalyzed by MoS2 and WS2 in surface states which did not chemisorb oxygen and were, according to XPS analysis, saturated by sulfide species. This is a clear experimental disproof of classical concepts that require coordinative unsaturation for catalytic reactions to occur on such surfaces. It supports new concepts developed on model catalysts and by theoretical calculations so far, which have been in need of confirmation from real catalysis.