Crude oils contain a variety of components-including asphaltenes-that can adsorb onto mineral surfaces and alter wetting.What distinguishes the asphaltenes from other constituents of an oil is their tendency to aggregate and even separate from the oil in response to changes in oil solvency. Because they change in size, asphaltenes can be viewed as both macromolecules and colloids. Their influence on wettability can change with this shift from molecular to colloidal regimes. As macromolecules, asphaltenes and other crude oil components with polar functionality can adsorb on mineral surfaces. Many different crude oils have been shown to have similar effects on wetting of dry silicate surfaces. When water is present, however, the results of exposing surfaces to different oils can be quite complex, depending on the distribution of water, the compositions of oil and brine, and mineralogy of rock surfaces. Acid and base numbers and the relationship between them provide a measure of the potential for a particular oil to alter wetting through ionic interactions, As colloids, asphaltenes can alter wetting by an additional mechanism. Near the onset of precipitation, wetting alteration occurs by surface precipitation because of the interfacial aggregation of the colloidal asphaltenes, which can precede flocculation in bulk. The influence of asphaltenes on wetting is thus strongly dependent on the environment in which they are found. Mixture refractive index is a useful measure for quantifying the stability of asphaltenes in a crude oil and thus in differentiating between macromolecular and colloidal contributions of asphaltenes to wetting alteration.