Mineral surfaces are important reactants. In sediments, and saturated soils, mineral surface area loadings can reach hundreds of square meters per liter. (Hydr)oxide minerals coat the walls of pipes used to distribute drinking water and to dissipate heat in cooling water systems. Concepts and information gained from homogeneous solution experiments provide the starting point for understanding interfacial reactions. There are, however, striking differences. Many surfaces acquire a net charge through adsorption of cations in excess of anions, or vice versa. This charge sets up a potential gradient in overlying solution which affects ion distribution in overlying solution. Surface-bound atoms are hydrated, and the ordering of water near mineral surfaces is different from what's observed in bulk solution. Surface-bound atoms have neighbors, yielding interesting cooperative and topological effects. These and additional phenomena can be explored by reacting mineral/water interfaces with structurally-varied organic probe molecules. Replacing one ligand donor group with another, adding or removing hydrophobic portions of molecules, and changing the placement of oxidizable or reducible moieties within the molecular structure can have surprising effects on pathways and rates of reactions at surfaces.