The effective area for mass transfer of the Sulzer structured packing Mellapak 125.Y, 250.Y, and 500.Y was measured under industrial-scale operating conditions, with a broad range of gas and liquid flows, in a pilot plant with a column having an internal diameter of 295 mm and a packing height of 420 mm. The experimental procedures and apparatus employed in this work were checked by making additional measurements of the effective area for 25-mm ceramic ring packing; these values were compared with literature data. This contribution shows that the effective mass-transfer area for packings 125.Y, 250.Y, and 500.Y can be considerably higher than the defined geometric area of the packing. The ratio of the effective area to the geometric area is a function of the Reynolds number defined with the liquid characteristic velocity and the wetted perimeter. The additional mass-transfer area beyond the fully wetted packing is attributed to liquid flow instabilities, leading to waves, film detachment, and droplet formation between the sheets. The effect is particularly important in Mellapak with low specific geometric area.