The effect of particle wettability on the hydrodynamics of gas-liquid-solid fluidized beds was studied in a cold-flow laboratory-model system. Average phase hold-ups in both three-phase and two-phase (freeboard) regions were measured in systems comprising hydrophobic or hydrophilic glass beads using water or 5 wt% aqueous solution of ethanol as the liquid phase, representing a coalescing (non-foaming) and a non-coalescing (foaming) system, respectively. Compared to hydrophilic particles, the lower gas hold-up observed with hydrophobic particles in the three-phase region is consistent with a greater average bubble size. This is explained by attachment of bubbles to hydrophobic particles and the formation of bubble-particle agglomerates with reduced ability to effect bubble break-up. Gas hold-up in the freeboard region is also lower in systems with hydrohobic particles, as is the steady foam thickness above the freeboard region in a foaming system. The experimental observations are quantitatively consistent with published models of the dynamics of foam formation. (C) 2020 Elsevier B.V. All rights reserved.