Flooding limits and pressure drop of gas-liquid countercurrent flow have been determined in internally finned tubes, representing a single channel of an internally finned monolith. It was found that with the appropriate liquid outlet geometry it is possible to achieve countercurrent annular flow at velocities that are relevant for hydrotreating of petroleum fractions. Comparison with an unfinned tube showed that the longitudinal fins inside the tube stabilize the liquid layer against slug formation, especially at high liquid velocities. The pressure drop was calculated using a countercurrent film flow model. By taking the extra pressure drop caused by the end-effects into account, it was possible to predict the pressure drop of both the finned and the unfinned tubes in the wavy annular flow regime.