The efficiency and capacity of valve trays in methanol-water distillation were studied in a 0.3-m-diameter column, covering a wide range of mixture compositions. The test results exhibit three unusual features. First, the efficiency was found to be substantially lower at both very high and very low methanol concentrations. Results indicated that the unique feature is closely related to the surface tension gradient, or the so-called "Marangoni effect". Second, the capacity of the valve tray was affected by impurities present in the methanol-water mixture. Methanol-water mixtures from an industrial methanol purification column had impurities foamed present and resulted in lower capacities. This too is attributable to the Marangoni effect. Third, the capacities of the valve trays were strongly dependent on the methanol concentration. At increased water concentrations, the flooding-point F factors increased from 4.0 to 6.5 (kg/m)(0.5)/s. The use of structured packing as a de-entrainment device (DED) between the trays increased the capacity substantially. The DEDs also enhanced the efficiencies at high F factors. Additional tests in a 0.3-m air-water column confirmed the effectiveness of the DEDs. It was also found that the DEDs reduced the pressure drop of the valve trays as a result of entrainment suppression.