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 liq. flows, in a pilot plant with a column having an internal diam. of 295 mm and a packing height of 420 mm. The exptl. procedures and app. employed in this work were checked by making addnl. 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 no. defined with the liq. characteristic velocity and the wetted perimeter. The addnl. mass-transfer area beyond the fully wetted packing is attributed to liq. 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. [on SciFinder (R)]