Humus existent in soil and water is often closely associated with clay minerals and forms special complexes. The adsorption of heavy metals on clay mineralhumus complexes is different from that on clay minerals or on humus. The authors investigated the adsorption and competition of Cu2+ and Cd2+ on montmorillonitehumic acid complexes in both the single system (Cu2+ or Cd2+) and the binary system (Cu2+and Cd2+). Experimental results show that, in the single system, the adsorption amounts of Cu2+on the complexes are higher than those of Cd2+ and, in particular, the adsorption amounts of both Cu2+and Cd2+increase linearly with the increasing of their initial concentrations. In the binary system, the adsorption amounts of Cu2+ are consistently a bit higher than those of Cd2+. This indicates that the selectivity of complexes for Cu2+ is better than that for Cd2+. The cation exchange process, which occurs between montmorillonite and Cu2+/Cd2+, may be a predominant mechanism in the process of adsorption. Whether in the single system or in the binary system, Cd2+is liable to replacing Ca2+which has the same electrovalence and a slightly larger radius than Cd2+, whereas Cu2+ is liable to replacing Na+ which has a similar radius and a lower electrovalence than Cu2+. Therefore, the adsorption amounts of Cu2+ on complexes in the binary system are obviously lower than those in the single system， while the adsorption amounts of Cd2+ are almost the same in both systems. These results might provide some clues to the migration and fixation of heavy metal ions in soil and water for further exploration.