The comparative behaviors of a micro-scale fixed bed multiphase reactor in up flow and down how modes of operation were studied fur hydrodesulfurization of atmospheric gas oil over a commercial catalyst. The experiments for the two modes of operation were conducted for a wide range of diluent size and other process variables such as liquid hourly space velocity and hydrogen/gas oil ratio. The results showed that the down how mode of operation using 0.19 mm size of diluent could be used for generatings reliable and meaningful data. On the other hand, the use of up flow mode of operation is restricted only for higher space velocities even while using 0.19 mm size of diluent. The results also indicated that the performance of the up flow mode of operation was poor at higher hydrogen/gas oil ratio. This change in performance is pronounced when higher size of diluent was used with the catalyst. In contrary, the hydrogen/gas oil ratio had minimum effect on the performance of the reactor for down flow mode of operation. (C) 2001 Elsevier Science B.V. All rights reserved.