High-energy ball milling has been employed to convert a microcrystalline, stoichiometric (S/Mo = 2), catalytically completely inactive MoS2 (prepared by high-temperature decomposition of ammonium tetrathiomolybdate, ATM) into an active catalyst, the activity profile of which was studied with test reactions (ethylene hydrogenation, H-2/D-2 scrambling, cis-trans isomerization of cis-but-2-ene, double-bond isomerization of 2-methyl-1-butene). Structural and surface properties of the materials were studied by XRD, SEM, TEM, XPS, nitrogen physisorption, oxygen chemisorption, and isotope exchange with D-2 (quantity of exchangeable surface hydrogen). The reaction rates obtained after mechanochemical activation were compared with data from a reference MoS2 made by low-temperature decomposition of ATM. Ball-milled MOS2 became active for most of the test reactions (except double-bond isomerization) only after reductive treatments that were effective also with the reference MOS2. After identical treatments, the ball-milled MoS2 was much more active in hydrogenation than the reference MoS2, whereas H-2/D-2 scrambling proceeded more slowly, and cis-trans isomerization not at all. On the basis of earlier conclusions about the site selectivity of these reactions, the activity pattern indicates that mechanochemical activation led to a site structure dominated by sites with multiple vacancies whereas single-vacancy sites were not present at all, which is in agreement with TEM results showing highly defective bent nanoslab structures after ball milling. The results confirm that ethylene hydrogenation, H-2/D-2 scrambling and cis-trans isomerization of cis-but-2-ene may be employed as test reactions for sites on MoS2 surfaces whereas data for the double-bond shift in 2-methyl-1-butene suggest that the Bronsted sites catalyzing this reaction are related to structural defects rather than to the regular MoS2 structure. (C) 2008 Elsevier Inc. All rights reserved.