Seawater samples of the sea surface microlayer were obtained from the ocean, coastal waters, and laboratory imitation experiments adopting glass plate, rotating drum, screen, and funnel samplers. The result was that surface microlayer samples of thickness 50 mum could be taken, not by the popular screen and funnel techniques, but by glass plate or rotating drum techniques. The layer of sudden change of physical and chemical properties in the surface microlayer was found at 50 mum below the sea-air interface. These physical and chemical properties included tens of physical and chemical parameters, such as concentrations of organic matter, nutrients, and dissolved trace metals, biochemical oxygen demand, chemical oxygen demand chlorophyll-alpha, surface tensions, and pH. Therefore, it was suggested that the layer of sudden change of physical and chemical properties in surface seawater should serve as a basis for defining the sea surface microlayer should, and be regarded as the practical operational thickness of the sea surface microlayer. The apparent sampling thickness of the sea surface microlayer from surface seawater should be 50 +/- 10 mum. (C) 2003 Elsevier Inc. All rights reserved.