The operation of a structured bed three-phase minireactor is studied when used for the hydrodesulfurization of a heavy gas oil fraction under real industrial conditions. This type of reactor consists of a spiral tube with an internal diameter a little larger than the catalyst extrudate diameter, in which the catalyst particles form a string bed. Four different loadings of the spiral reactor were realized for the desulfurization of the gas oil fraction in a variety of operating conditions. The effect of the catalyst mass, string bed length and the fluid velocities on the performance of the spiral reactor concept is examined. The stability as well as the repeatability of the spiral reactor operation and the loading procedure of the string bed was tested. The influence of reaction temperature and the gas-to-liquid ratio on the spiral reactor performance was also investigated. The performance of the spiral reactor was compared with that of a convectional miniscale diluted bed reactor as well as with a pilot reactor. From the results presented it is concluded that the operation of the string bed spiral reactor is quite satisfactory and the impact of the catalyst mass and the bed length on its performance is negligible.