H-2/D-2 exchange (473-583 K), 1.3-butadiene hydrogenation (418-513 K) and tetrahydrothiophen hydrodesulphurisation (428-557 K) have been studied over powdered Co9S8 (surface area, 7 m(2) g(-1)) using D-2 as an isotopic tracer. Hydrogen exchange proceeded as a first order process at a modest rate (k(540) = 1.0 h(-1) m(-2)) with an apparent activation energy of 67 kJ mol(-1). Butadiene hydrogenation was diagnostic as to the surface state of Co9S8: samples showed either predominant 1:2-addition or 1:4-addition of hydrogen, interpreted as indicating the presence in the surface of single sites or pair/ensemble sites, respectively. Reactions at 473 K in the presence of D-2 gave butenes containing 0-6 D-atoms: exchange patterns obtained from these D-distributions showed that a proportion of butadiene molecules underwent extensive dehydrogenation during the normal progress of hydrogenation. At 633 K this dehydrogenation activity was evident as self-hydroge nation which occurred in the absence of D,. Tetrahydrothiophen was desulphurised in the presence of D-2 to thiophen (void of D), butadiene (containing 0-5 D-atoms) and 1-butene (containing mostly 0 and 4 D-atoms). Increase in temperature or in deuterium pressure favoured butene formation so that it became the dominant product (88%). Tetrahydrothiophen also underwent self-hydrodesulphurisation in the absence of D-2, A mechanism is proposed, consistent with this D-tracer information, that accommodates dehydrogenation, desulphurisation and hydrogenation steps in the overall process. The activity of powdered Co9S8 exceeded that of powdered MoS2. (c) 2006 Published by Elsevier B.V.