We call attention to the possible importance of pH-dependent speciation in determining the distribution coefficient of americium in a granitic ground water. We obtained experimental distribution coefficients for americium. We used the geochemical code PHREEQE to calculate the amount present of americium complexes with hydroxides and carbonates over a range of pH, using formation constants taken from the literature and one estimated for AmOHCO3, a neutral complex suspected of playing an important role in the aqueous chemistry of americium, although not experimentally evaluated. Combining the above inputs, we calculated individual distribution coefficients for americium complexes. We are able to show that the batch distribution coefficient for americium can be seen as the sum of individual distribution coefficients of various complexes.