Colloidal gas aphrons (CGAs) are micron-sized gas bubbles produced by stirring surfactant solutions at high speed. A single CGA dispersed in water is composed of a gaseous inner core, surrounded by double water-soapy layer. CGAs have large interfacial area per unit volume and exhibit relatively high stability. These characteristics make CGAs very suitable in flotation systems. Early work on CGA has shown that it has considerable potential in the field of solvent extraction. While the area of application is very broad, is has been used for the flotation of biological products such as microbial cells, removal of ash-forming materials from coal, waste water processing, etc. In this paper, the flotation of magnesium hydroxide (particle size 2.3-9 mu m) and polystyrene (particle size 24-300 mu m) by CGAs has been studied. The effect of particles size on removal efficiency was analyzed. There is an optimum particle and bubble size that will give high removal efficiency. The effect of surfactants type and surfactants concentration on removal efficiency has also been investigated. Results of more than 100 experiments show that CGAs made from ionic surfactants (HTAB, SDBS) is more efficient for the removal of fine particles than that generated from non-ionic surfactant (Tween 20). High removal efficiency can be obtained at surfactant concentration approximately equal to the critical micelle concentration (CMC). Yields higher than 50% of magnesium hydroxide and 97% of polystyrene are reached at surfactant CMCs. (C) 2005 Elsevier B.V. All rights reserved.