The mechanisms governing the adsorption of a commercial humic acid (HA) on a kaolinitic clay were determined. The amount of HA fixed increased with the ionic strength of the medium and the presence of divalent ions (Ca2+). The effect of Ca2+ is due to its ability to establish (1) intramolecular bridges causing condensation of the HA, and (2) intermolecular bridges between the clay and the HA. A stable clay-humic complex containing only the fraction of HA that withstands washing with water was prepared and characterized by high-resolution Ar adsorption volumetry. The overall results show that (1) HA interacts strongly with the basal surfaces of the clay, (2) Ca serves to establish Ca2+ bridges between HA and clay, and (3) HA and clay interact via at least two distinct adsorption mechanisms.