The limiting factors on the flooding performance of monoliths have been investigated in detail. In addition to the channel size and the void fraction of the monolith itself, the flooding limits are strongly impacted by inlet and outlet effects of the reactor bed as well as the stacking of monolith segments. Solutions to overcome these additional hydraulic restrictions, based on stacking slices of high-void-fraction monoliths with strategic step changes in cell density, are identified and evaluated. Implementation of the improvements allowed for the countercurrent operation of monoliths with a channel size as small as 1.25 mm even when a high-surface-tension liquid (water) is used. With the enhanced configurations, flooding experiments over a wide range of channel geometries of single and multiple monoliths were performed. Results from small-scale testing are in fair agreement with those obtained from the large-diameter tests, indicating a good scale-up behavior of the technology. The flooding limits are conveniently described with a flooding correlation based on an adjusted capacity plot taking the geometry properties of the monolith into account: C-G/(is an element ofrootgd(h)) = -0.025 + 0.12F(LG)(-0.475) for 0.1 less than or equal to F-LG less than or equal to 10. The measured flooding performance is in good agreement with recently reported data for flooding investigations of a falling film in an annular-flow configuration (Stockfleth, R.; Brunner, G. Ind. Eng. Chem. Res. 2001, 40, 6014-6020) that were performed with a significantly larger hydraulic diameter (12.5 mm). Monoliths enable stable countercurrent film flow operation with capillary-sized channels (1.25-4.0 mm), which so far was not feasible for any other packing structure. The findings are summarized in a design guide for the configuration of a monolith reactor for countercurrent operation.