In view of its widespread application in aviation turbine fuel, diethyleneglycol monomethylether (DiEGME) and its interactions with water and n-heptane have been characterized using turbidity, interfacial tension, water activity, and water absorption measurements. This additive has been implicated in a number of problems in recent years, which have arguably arisen from its various physicochemical interactions with fuel and fuel system components, for which few data were hitherto available. The present study has therefore addressed the more fundamental aspects underlying such interactions using n-heptane as the hydrocarbon. Turbidity results indicate an increased level of water solubilization, owing to the formation of DiEGME-water clusters (similar to 1:8 ratio), as the DiEGME concentration exceeds its specification maximum value of 0.15% (w/v) in fuel. Interestingly, this same composition is found in separated water, resulting from additive partitioning from fuel, leading to similar to 50% DiEGME/water mixtures. The combined use of interfacial tension, water activity and absorption measurements, and solubility parameters is able to explain this tendency as being due to a reduction in water activity in the presence of DiEGME, with this latter property being reduced significantly above 50% DiEGME, which therefore appears to be the most thermodynamically stable composition. Water activity considerations also provide the basis for understanding the action of DiEGME as a thermodynamic icing inhibitor, consistent with the role that hydrogen bonding plays in reducing water activity and in line with the wate-ractivity-based ice nucleation theory (Koop, T.; Luo, B.; Tsias, A.; Peter, T. Nature 2000, 406, 611-614). Correspondingly, the thermodynamic activity of DiEGME, derived herein using a Gibbs-Duhem treatment of water activity data, is shown to be reduced in the presence of low levels of water, which would be sufficient to restrict the fuel solubility of this material as observed in practice.