The influence of the configuration of additional void space (AVS) on the flooding point and the gas-liquid mass transfer coefficient (k(L)a) of structured packings in a countercurrent trickle-bed reactor has been investigated. Experiments have been performed with six structured packings with different AVS configurations. The experiments have been performed with air, water and 3 mm glass beads in a Perspex model system. A clear difference can be observed between continuous and discontinuous AVS configurations; only continuous AVS configurations are shifting the flooding boundaries to higher gas and liquid fluxes. The k(L)a of all structured packings with AVS is comparable to the k(L)a of a cocurrent packed bed. Flooding in structured packings is qualitatively described by identifying three types of liquid flow. These flows can be manipulated with the configuration of the AVS and therefore the flooding point can be influenced. By applying AVS, flooding can also be prevented completely without loss in mass transfer (k(L)a). (C) 2007 Elsevier B.V. All rights reserved.