It is well established that pressure drop and liquid holdup under trickle flow conditions are functions of the flow history. However, the extent of possible variation of these and other critical hydrodynamic parameters has not been fully quantified. In this study, specifically defined prewetting procedures are used as limiting cases for hydrodynamic hysteresis. These are: Non-prewetted. Levee prewetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flows are introduced. Kan(L) prewetted: the bed is operated in the pulse flow regime (by increasing liquid velocity) after which liquid flow rate is reduced to the desired set point (all at the desired gas flow rate). Kan(G) prewetted: the bed is operated in the pulse flow regime (by increasing gas velocity) after which gas flow rate is reduced to the desired set point (all at the desired liquid flow rate). Super prewetted: the bed is flooded and gas and liquid flows are introduced once draining commences. It is shown that the upper limiting case for pressure drop is the Kan(L) mode of operation. The lower limiting cases are the non-prewetted and Levee prewetted modes (these coincide). Pressure drop may vary by as much as 700% even for prewetted beds. Liquid holdup is different in all five prewetting modes. The upper limiting case is the Kan(G) mode of operation, while the lower limiting case is the non-prewetted mode (Kan(G) holdup is approximately 160% that of non-prewetted mode holdup at U-G = 90 mm/s). At low gas velocities the Kan(L), holdup can be 400% of that of the non-prewetted beds. Importantly, the lower limiting case for prewetted beds is the Levec mode. Holdup in the Kan(G) mode may be as much as 130% of the holdup in the Levee mode (at U-G = 90 mm/s). The effect of hydrodynamic multiplicity of the volumetric mass transfer coefficient is measured by the desorption of oxygen from water into nitrogen. In this case the different prewetting procedures result in three distinct regions, the upper region being the Kan and Super prewetted beds, the intermediate region being the Levee prewetted bed and the lower region being the dry bed. Mass transfer coefficients in the upper region can be as much as 600% of that of the lower region and 250% of that of the intermediate region. Evidently, prewetting (and even pulsing flow prewetting) does not guarantee that the bed is operating at the maximum values of pressure drop, holdup and mass transfer coefficient. Evidence of operation in between the limiting cases is presented. These non-limiting cases can be reached in multiple ways. (c) 2006 Elsevier Ltd. All rights reserved.