Oily wastewater cleanup can be carried out by gas flotation. When properly operated gas flotation units can reduce oil concentrations of wastewater effluents to well below 40 mg/l. Gas flotation is particularly valuable for heavy oils (oils having a density close to that of water). The flotation process relies on the attachment of gas bubbles to the dispersed oil droplets. This attachment is heavily dependent on the complex processes involving the surface characteristics of the oil droplets and their interaction with gas, and can only be optimally achieved if the surface science conditions are properly understood. The attachment mechanisms include the oil/bubble contact, the interactions of chemical additives (usually surfactants) in aiding this contact and the spreading of the oil around the gas bubble. Additionally, initial agglomeration of the oil emulsion droplets is needed to increase the droplet size to within the range needed for effective flotation, similar to 60 pm. This paper examines the essential surface science of the gas flotation process, particularly the gas attachment to oil droplets and the use of surfactants. We discuss the stages of attachment of the gas bubble to the oil droplet, and provide further photographic evidence concerning the importance of the spreading of the oil around the gas bubble for gas flotation. (C) 2003 Elsevier B.V. All rights reserved.