The Dependence of Hydrodynamic Parameters, Such as Pressure Drop, on the Flow History of the Bed is Called Hysteresis. This Phenomenon is Most Commonly Associated with Changes of Flow Distribution and Flow Patterns with the Flow History. Many Studies Have Shown that Increased Operating Pressure Will Affect Flow Distribution and Wetting Efficiency; However, There Seems to Be No Study of the Effect of the Elevated Operating Pressure on the Extent of Hysteresis. in This Study, an Experimental Investigation of the Hysteresis in a High-Pressure Trickle Bed Has Been Performed. a Hysteresis Factor Has Been Introduced to Quantify the Extent of Hysteresis and Was Found to Be a Strong Function of Gas and Liquid Operating Flow Rates as Well as the Operating Pressure. in the Region of Lower Liquid Velocities, Hysteresis is Present Regardless of the Operating Pressure or Gas Velocity. in the Region of Higher Liquid Velocities, Increases in Both Pressure and Gas Velocity Will Lower the Extent of Hysteresis. for the Range of Conditions Considered in This Study, the Extent of Hysteresis Was Uniquely Determined by the Pressure Drop in the Levee Mode, Regardless of the Operating Pressure or Velocities. the Results Are Interpreted in Terms of the Phenomenological Analysis of Al-Dahhan and Dudukovic (Al-Dahhan, M. H.; Dudukovic, M. P. Chem. Eng. Sci. 1994, 49, 5681-98), Which Relates Operating Pressure and Gas Velocity to the Flow Distribution and Wetting Efficiency in a Trickle Bed Reactor. © 2008 American Chemical Society.