Concentrations of labile and total Al in soil extracts were measured as a function of equilibrium solution pH in six forest soil organic horizons acidified with HNO, (0-20 cmol H+.kg-1) under controlled conditions of ionic strength (0.05 M NaNO3), temperature (23 C), and solution:soil ratio (25:1). Decreases of 0.1-0.2 in solution pH in the range of pH 2.4-4.5 resulted in increases and decreases in concentrations of labile and total Al. Organic acids and soluble C were the dominant Al-complexing ligands in solution,and acidification of each horizon decreased their concentration. Silicon and F concentrations in the equilibrium solutions were lower than that of C, but levels were similar to those of nonlabile Al. Nonlabile complexes were the dominant forms of soluble Al in horizons above pH 4, and labile Al comprised an increasing percentage of total Al as pH decreased, especially below pH 3.