Solubility and kinetic data indicated that concentrations of aluminum (Al) extracted with 1MKCl are determined by the solubility of a precipitated A1(OH)3phase in soils dominated by variable charge minerals. Kinetic studies examining the release of Al on non‐treated and KCl treated residues indicated the precipitation of an acid‐labile Al phase during the extraction procedure. The log ion activity products estimated for the KCl extracts ranged between 8.1–8.6 for the reaction Al(OH)3+ 3H+< = > Al3++ 3H2O, which was similar to the solubility product of several Al(OH)3phases. The mechanism proposed for Al precipitation indicated that Al released by exchange with added K+hydrolyzed and released H+that was readily adsorbed on surfaces of variable charge minerals. The increased ionic strength of the extracting solution further increased the amount of H+adsorbed to the variable charge surface and reduced the H+concentration in the aqueous phase. Consumption of H+induced further hydrolysis of Al, resulting in supersaturation of the extracting solution and formation of polynuclear hydroxy Al species. It was concluded that the 1MKCl extraction does not quantitatively extract salt exchangeable Al from variable‐charge soils.