The effects of fluorine and nickel on the hydrodenitrogenation of o-toluidine on alumina-supported tungsten catalysts were studied in a continuous-flow reactor at 320 to 370 degreesC and 3.0 MPa. The catalysts were prepared from ammonium tetrathio-tungstate and were fully sulfided. The kinetic data were obtained by varying the initial partial pressure of the reactant and the reaction temperature; Langmuir-Hinshelwood models were used to fit the kinetic data. The simultaneous reactions of cyclohexene and o-toluidine enabled us to study the inhibition effect of o-toluidine on the hydrogenation of cyclohexene and to determine the difference between the sites for these reactions. The kinetic data suggest that the formation of toluene and 2-methylcyclohexylamine from o-toluidine occurs through a common partially hydrogenated intermediate, dihydrotoluidine, which leads to toluene by elimination and 2-methylcyclohexylamine after further hydrogenation. Fluorination changes neither the activation energies for the hydrogenation reaction nor the heat of adsorption of o-toluidine. Fluorine thus does not change the intrinsic properties of the active sites but does affect the apparent activity by influencing the number of active sites. Only a change in the morphology of the metal sulfide surface, by stacking, can explain the fluorine effect. Addition of nickel changes the nature of the active site by creating a nickel-associated sulfur vacancy, which is highly active for hydrogenation.