MoS2 catalysts used in slurry-phase hydroconversion of bitumen deactivate when the solid coke catalyst recovered from the product is recycled in a semi-batch reactor operated at high residue conversion (445 degrees C and 13.8 MPa H-2). The catalyst deactivation, manifested by an increasing coke yield, is dependent upon the MoS2 concentration in the recycled coke catalyst and the age of the coke in the reactor. Characterization data show significant changes in the chemical and physical properties of the coke when recycled under hydroconversion conditions. In particular, with increased recycling, the coke becomes more graphitic with a decreased H/C ratio and an increased aromatic/aliphatic carbon ratio. Model ex situ coke aging experiments, conducted in He at 700 degrees C for 15 h, were used to confirm that changes in the chemical properties of the recovered coke determined the extent of catalyst deactivation, whereas morphological changes were less important. On the basis of these results, a model of the catalyst deactivation mechanism during recycling is discussed.