The hydrodynamic behavior of countercurrent gas-liquid flow and the gas-liquid mass transfer characteristics have been investigated in an internally finned monolith. Special attention has been paid to the flooding limits, the pressure drop, and the liquid hold-up. Gas-liquid mass transfer has been investigated experimentally and the influence of the liquid and gas flow rate has been determined. From the flooding experiments it can be concluded that countercurrent flow is possible at industrially relevant flow rates and that the limitations are mainly engendered by the liquid outlet. The flooding correlation of Wallis can be used to describe the flooding data. In the wavy annular how regime, below the flooding limits, both pressure drop and liquid hold-up can be predicted by a model that is based on a theoretical analysis of the momentum transfer. It was found that the deviations between the model and the experimental results are to a large extent induced by inlet effects and only moderately determined by interfacial waves. The mass transfer measurements showed that k(GL)a(GL), which ranged from 0.03 to 0.16 s(-1), was much higher in the upper part of the monolith. The influence of the liquid flow rate becomes significant at higher liquid how rates while the gas flow rate only has a moderate effect. Developing waves probably cause some extra mixing in the liquid film resulting in improved mass transfer.