Desulfurization and hydrogenation of aromatics in diesel oil were investigated in an isothermally operated trickle-bed reactor 500 mm long and with a 19-mm ID, using commercial bifunctional NiMo/Al2O3 catalysts. The operating temperature, pressure, liquid hourly space velocity, gas/oil ratio, and the initial concentration of H2S were 300-380degreesC 2-8 MPa, 1-4 h(-1), 100-500 m(NTP)(3)/m(3) and 0-8 vol %, respectively. The increase in pressure and decrease in liquid hourly space velocity lead to deeper desulfurization and hydrogenation of both aromatics. Although a higher gas-to-oil ratio and lower initial concentration of H2S enhance the desulfurization reaction, these parameters play almost no role for hydrogenation. Desulfurization increases sharply with increased temperature, but with the same change in temperature the conversion of aromatics increases only up to 360degreesC above which it falls sharply. This behavior is explained by the approaching of chemical equilibria by the reversible hydrogenation reactions of the aromatics at higher temperatures. A mechanistic mathematical model was developed for a two phase flow reactor, considering both mass transfer and chemical reaction in the reactor. The kinetic equations for desulfurization and for the hydrogenation of mono-, di- and polyaromatics were established. Simulated results are compared satisfactorily with the experimental observations.