The performance of a representative sheet metal structured packing (Mellapak 250.Y) in commercial-scale experimental columns (1.2 m i) is presented over a wide range of test systems and conditions. Pressures span the range of 0.02-27.6 bar. Systems include o-xylene/p-xylene, cyclohexane/n-heptane, and i-butane/n-butane. The operations were carried out in two separate columns (high and low pressure) utilizing two identical loads of packing. Internal bed samplers were utilized. Capacity, separation efficiency, and pressure drop are reported and compared to the predictions of published models. Some of the more important results were obtained at the extreme ends of the operating range. It was found that, with an appropriate distributor, separation efficiency is maintained down to extremely low liquid rates, indicating that dewetting is not significant; At the other end of the operating range (high pressure and high liquid rate) a "hump" or region of decreased mass-transfer performance was observed at the start of the loading region. Published models perform reasonably at moderate pressures, but performance deteriorates at higher pressure. Models also break down at low liquid rates where, as noted above, performance does not fall off as predicted.