An attempt has been made to assess the involvement of the acidic functional groups of humic substances in the formation of clay-humus complexes. The humic substances used included four different samples of varying acid-group content as well as a series of methylated derivatives of one humic acid (HA). Amounts of soluble HA/FA (fulvic acid) retained by montmorillonite after repeated washing of the predried complexes with water were estimated. Results reveal a trend of decrease in fixation with increase in acidity of the humic samples. Methylation of the acidic groups has the same effect. It is proposed that a decreasing content of acidic groups in humic substances promotes their fixation by montmorillonite because of a lowering of interparticle repulsive forces as well as a reduction in hydration energies; the net energy available for breaking down the clay-humus bonds is consequently decreased. Nonionic attractive forces are important interactive modes in complexes that have been dried. © 1990 Williams & Wilkins.