Catalytic properties of unsupported MoS2 catalysts in the thiophene hydrodesulfurization reaction were determined in the temperature range 623-653 K. The catalysts were prepared by ex situ decomposition of ammonium thiomolybdate (ATM) crystals in a mixture of 15% H2S in H-2 at 673 K. Activity of catalysts decreased very rapidly before reaching a steady state after 15 h on-stream. The thiophene conversion went down from 10-12 to 3-4% in that time. The surface area of the catalysts also decreased during the catalytic reaction from 40-50 to 8-10 m(2)/g. Selectivity for hydrodesulfurization, hydrogenation and isomerization reactions was affected distinctly by the deactivation process. By increasing the reaction time, double-bond isomerization increased, hydrogenation of butenes decreased and hydrodesulfurization remained constant. Results indicate that the main cause of activity decay was surface area loss that was due to sintering of MoS2 crystallites. Selectivity variation indicates that different active sites are involved for the three reactions. A deactivation model involving diminution of active sites located in edge and rim sites of small MoS2 particles is proposed to explain the variation of product distribution.