Diesel engines are widely employed as the power sources for in-land and marine transportation and industrial power plants. However, pollutants emitted from diesel engines, especially nitrogen oxides and particulate matter, which can be detrimental to human health and to the ecological environment, attract much research interest. Although the emulsion of diesel fuel has been used to reduce engine emission, most previous relevant applications are still restricted to two-phase emulsions. Three-phase emulsions are generally applied in pharmaceutical, cosmetic, and food industry, but their usage as an alternative diesel fuel has not been found. The chemical agent diglyme, which is used as an oxygenated additive to diesel fuel, improves combustion characteristics of diesel engines, boilers, and furnaces. The effects of oxygenated agent addition on the emulsification properties of three-phase emulsions have never been studied. Hence, the emulsification properties of multi-phase emulsion of oil-in-water-in-oil (O/W/O) type added with an oxygenated agent are investigated in this study. For comparison purposes, the emulsification characteristics of two-phase oil-in-water (O/W) and water-in-oil (W/O) emulsions with diglyme additive were analyzed as well. Experimental results show that the viscosities of O/W and W/O two-phase emulsions increase with the increase of their inner phase content and the addition of an oxygenated agent. The addition of diglyme agent to the emulsions deteriorates the emulsification activity (EA) and emulsification stability (ES) of W/O and O/W/O. This is particularly apparent in the latter case. The presence of the oxygenated agent in the water phase, rather than the oil phase, reduces EA and ES more severely. On the contrary, both EA and ES increase with the addition of an oxygenated agent, especially when the oxygenated agent is added to the water phase for O/W emulsions. (C) 2004 Elsevier Ltd. All rights reserved.