Nuclear magnetic resonance imaging (MRI) has been applied to study the phase distribution in the monolith film flow reactor. The accumulation of the liquid in the corners of the square channel with an arc-shaped gas-liquid interface has been determined. The average liquid saturation is in good agreement with model calculations. Non-uniformities of the liquid distribution over the four corners of the square channel were apparent, besides the maldistribution over the cross-section of the monolith. Computational fluid dynamics (CFD) calculations applying the measured liquid distribution predict a broadening of the residence time distribution and a shot-ter break-through time, due to the maldistribution, which is in very good agreement with experimental results, The impact on the modeled gas-liquid mass transfer performance seems to be negligible, due to the nearly linear relation between k(GL)a(V) and u(Ls).