The three-levels-of-porosity (TLP) reactor is a three-phase reactor suitable for countercurrent trickle-Row. Its packing consists of permeable catalyst baskets which are arl arranged in the reactor, The volumetric gas-liquid mass transfer coefficient (k(L)a) was measured as a function of liquid now for five different TLP configurations. Gas-liquid mass transfer occurs at the Faces of the catalyst baskets and at the liquid jets falling from basket to basket. For the measured configurations, the basket faces contributed to only 16% of the overall mass transfer. The mass transfer to the faces of the baskets is in good agreement with values calculated from theoretical models. The mass transfer lu the jets is three times higher than predicted by the penetration theory for stagnant media. This is attributed to small waves at the jet surface, internal circulation and entrainment effects for the jets colliding with the baskets. The TLP reactor has a high freedom in design, which makes it possible to adapt the mass transfer rate to the reaction rate of a catalytic process.