The flooding performance of square channel monoliths for a wide range of geometric parameters and liquid properties has been evaluated. Hydrodynamic flooding is determined by a strong increase in pressure drop for increasing gas flow rate. Inefficient drainage of the liquid at the outlet of the monolith as well as liquids with high surface tension and monoliths with smaller channel sizes lower the flooding limits significantly, indicating a dominating impact of capillary forces. Increased liquid viscosity lowers the flooding limits, but the effect is less pronounced. The column diameter has a rather negligible effect on flooding. For higher liquid flow rates, a hysteresis effect was observed, shifting the deflooding gas flow rates to lower values. Misaligned stacking of monolith sections leads to a decrease of the flooding limits (approximately 15%), independent of the number of stacks. Common flooding correlations need to be adapted with the Bond number to describe the experimental data. For the type of outlet device applied in this work and the alignment of the outlet device with the channel walls, the experimental data are well correlated by C-F,C-0.25,C-0/Bd = [204.4 + 297.1F(LG)(1.5) - 171.4e(-FLG)](-1) for d(h)/l(c) > 1.15. A comparison with the requirements of applications revealed that channel sizes of 2.9 and 4.1 mm (50 and 25 cpsi) are well-suited for hydrotreating and reactive stripping operations. For the more demanding needs of catalytic distillation, the larger channel size is more favorable.