specific component and increases the volume fraction of one or both of the two other components. Each point within the triangle represents a possible composition of a mixture of the three components or pseudo-components, which may consist (ideally, according to the Gibbs' phase rule) of one, two or three phases. These points combine to form regions with boundaries between them, which represent the "phase behavior" of the system at constant temperature and pressure. The Gibbs phase diagram, however, is an empirical visual observation of the state of the system and may, or may not express the true number of phases within a given composition. Apparently clear single phase formulations can still consist of multiple iso-tropic phases (e.g. the apparently clear heptane/AOT/water microemulsions consist multiple phases). Since these systems can be in equilibrium with other phases, many systems, especially those with high volume fractions of both the two imiscible phases, can be easily destabilised by anything that changes this equilibrium e.g. high or low temperature or addition of surface tension modifying agents. However, examples of relatively stable microemulsions can be found. It is believed that the mechanism for removing acid build up in car engine oils involves low water phase volume, water-in-oil (w/o) microemulsions. Theoretically, transport of the aqueous acid droplets through the engine oil to microdispersed calcium carbonate particles in the oil should be most efficient when the aqueous droplets are small enough to transport a single hydrogen ion (the smaller the droplets, the greater the number of acid water droplets, the faster the neutralisation). Such microemulsions are probably very stable across a reasonably wide range of elevated temperatures. References Bibliography Prince, Leon M., Microemulsions in Theory and Practice Academic Press (1977) . Rosano, Henri L and Clausse, Marc, eds., Microemulsion Systems (Surfactant Science Series) Marcel Dekker, Inc. (1987) Colloidal chemistry Soft matter