A systematic experimental work for evaluating isothermality of a pilot reactor, flow regime, axial dispersion, and mass transfer gradients between the catalyst surface and the bulk fluid in a trickle-bed hydrotreating reactor is presented. Experiments were carried out at the following operating conditions: 5.3 MPa total pressure, 356.2 mL mL(-1) hydrogen-to-oil ratio, 613-653 K reaction temperature, and 1.0-2.5 h(-1) liquid hourly space velocity. Commercial NiMo catalyst and feedstock were used for hydrotreating reactions. The catalyst (2.3 mm diameter) bed was diluted with silicon carbide (1.4 mm diameter) with an inert-to-catalyst volume ratio of 1. Various. criteria reported in the literature were employed for evaluating axial dispersion in liquid and gas phases. Two typical tests were employed to determine mass transfer gradients: (1) changing the amount of catalyst loaded to the reactor and the feedstock flow rate at constant LHSV and (2) conducting experiments using different amounts of catalyst at variable LHSV.