Residence time distributions (RTDs) were measured for trickle flow in a bed of glass beads at different liquid flow rates and hydrodynamic states to obtain liquid holdup and axial dispersion measurements using the piston-dispersion (PD) model. The different hydrodynamic states were achieved by means of different pre-wetting procedures. It was found that all dispersion data show the same relationship to interstitial velocity, independent of the employed pre-wetting procedure. This relationship strongly reminds of Aris-Taylor type dispersion found in laminar flow through tubes. Based on the analogy of laminar liquid flow through a set of parallel tubes, a theoretical relationship between partial wetting, dispersion and liquid flow rate can be determined. Wetting efficiencies calculated from this relationship agree reasonably well with existing correlations. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.