The Complex formation of the humic acid (isolated from ground water) with trivalent metal, americium, has been examined by absorption spectroscopy at varing pH-value, 6.0 to 3.0. In the study of the complexation, the Proton Exchange Capacity (PEC) of the humic acid was characterized. And the loading capacity of the humic acid for the metal ion was determined separately at each pH. The loading capacities were reduced with reducing pH. The description of the complexation of humic acid with metal was based on a model as simple as possible. Taking the loading capacity into account, the humate complexation constant is found to be independent of pH. The overall complexation constants at every pH value lie between 6.02 and 6.33 in 0.1 M NaClO₄.