Macromolecules in the pore fluid of a soil may influence the mobility of hydrophobic compounds by their partitioning to the macromolecule, which moves with, or even faster than, the water. The mobility is described mathematically by a chemical transport model. The significance of the model was demonstrated experimentally in a ground-water infiltration microcosm using dextran, a polysaccharide, as the macromolecule added to lake water pumped through soil columns. Breakthrough curves showed that 500mg/l of dextran enhanced the mobility of hexachlorobenzene by approximately 25 percent. With more hydrophobic macromolecules, such as humic acids, the theory predicts a significantly larger mobility enhancement, a phenomenon that may explain the presence of very hydrophobic pollutants in deep ground-water aquifers.