The wettability of a subsurface system contaminated by coal tar or creosote is a major determinant of the capillary forces and, hence, the distribution and recoverability of these contaminants. The objective of this paper was to identify mechanisms that control the wetting characteristics of coal tar and creosote in the subsurface. Asphaltenes separated from these nonaqueous phase liquids were mixed with two different aromatic solvents and used in adhesion and atomic force microscopy tests. When asphaltene concentrations were low (< 37.5 g/L), electrostatic double-layer and van der Waals interactions were predominantly responsible for changes in the wetting behavior irrespective of the solvent characteristics. In this case, oil-wetting conditions were observed at pHs below and slightly above the point of zero charge (similar to4-5) of the DNAPLs. At higher asphaltene concentrations (similar to 150 g/L), asphaltenes were observed to precipitate on the quartz slide in aggregates as large as 3500 run. The deposition of these particles on a solid surface rendered the surface oil wetting regardless of the electrostatic repulsive forces. Aggregation and precipitation occurred at lower asphaltene concentrations in toluene in comparison with the mixed naphthalene solvent due to differences in their solvation power.