Effective packing areas of Sulzer Mellapak 250Y and 500Y structured packings were determined in a 0.46-m OD packed column. A chemical method (absorption Of CO2 from air using 0.1 kmol/m(3) NaOH) was employed for the measurements. The packing performance was reported as fractional area, defined as effective area normalized by specific packing area. Under high surface tension conditions (similar to 72 mN/m), the fractional area decreased by approximately a factor of 2 in going from the 250Y to the 500Y packing. When similar tests were conducted at reduced surface tensions (similar to 35 mN/m, achieved by the addition of surfactant), the fractional area of the 250Y was unchanged, whereas that of the 500Y increased by 50% relative to the base case. The results indicate that, at high surface tension, access to the surface of the 500Y packing was being inhibited. Lowering the surface tension served to maximize the effective area of the packing.