Gas-liquid mass transfer has been determined in internally finned monoliths (IFM) by desorbing oxygen from a falling water film. Measurements were performed for various monolith lengths to quantify the inlet effects caused, among others, by the development of the concentration profile. The local volumetric mass transfer coefficients were found to vary from approximately 0.04 (s(-1)) at the inlet to about 0.01 (s(-1)) in the downstream sections. A theoretical model has been derived, assuming that the water flow can be treated as a film falling down in a corner. By averaging the radial mass transfer perpendicular to the film surface, mass transfer can be predicted within reasonable accuracy. On comparing the film in the IFM with a film along a vertical plane, it was found that in an IFM the concentration profile develops faster while mass transfer is less effective in the developed sections.