The hydrogenation reactivity of some aromatic compounds used to model diesel fractions was examined under sulfur-free conditions. Reactions of toluene, 1,2,3,4-tetrahydronaphthalene (tetralin), and naphthalene, separately and as mixtures, were studied using a commercial Ni/Al2O3 catalyst. The reactivity decreased in the following order: naphthalene much greater than tetralin > toluene. Because of competitive adsorption and subsequent inhibition, naphthalene severely reduced the other hydrogenation rates in mixtures, whereas the hydrogenation rate of naphthalene was little affected by the concentration of toluene or tetralin. A kinetic model based on the general form of the Langmuir-Hinshelwood equation was developed. The activation energies of toluene, tetralin, and naphthalene were found to be 52.9, 40.4, and 58.7 kJ/mol, respectively, and the reaction orders of the monoaromatic and diaromatic compounds were about 1.4 and 2.1, respectively. Furthermore, simulations showed that the reaction kinetics in mixtures can be successfully described with models based on single-compound experiments, if surface-concentration terms (K(i)c(i)) for all aromatics are included in the rate equations.