Comparison of laboratory trickle-bed and up-flow reactors over a range of operating conditions, which cover both gas and liquid reactant limitations, has been investigated using hydrogenation of alpha-methylstyrene to cumene in a hexane solvent over 2.5% Pd on alumina extrudate catalyst as a test reaction. The results show that when the reaction is gas limited at low pressure and high liquid feed concentration, trickle bed reactor outperforms the upflow reactor. At high pressure and low liquid feed concentration the reaction becomes liquid limited and upflow reactor performs better. It is concluded that the advantage of upflow or downflow depends on the reaction system type (i.e. whether the reaction is Liquid or gas limited). A single criterion for identifying the limiting reactant is proposed which can explain most of the data reported in the literature on these reactors. Comparison of the experimental observations and the predictions of the reactor scale and pellet scale models available in the literature is presented.