Five kinds of low-molecular-weight organic acids were chosen for investigating the effect of organic acids on Al release from kaolinite.The results showed that organic acids could accelerate the release of Al from kaolinite due to formation of Al-organic acid complexes.Organic acids increased the soluble Al in the order oxalic acid> citric acid> malonic acid> malic acid> lactic acid.The ability of releasing soluble Al of oxalic acid, citric acid and lact ic acid decreased with the increase of pH, and the effect of pH on the release of soluble Al in oxalic acid system was the most remarkable, whereas the soluble Al increased with pH in malic acid system.The enhanced release of soluble Al by organic acids increased with the increase in concentration of the organic acids.The amount of exchangeable Al in the systems of malic acid and lact ic acid at lower pH(pH 3.5 and pH 4.0) was larger while that at higher pH(pH 4.5 and pH 5.0) was smaller than that in the control system.Citric acid decreased the exchangeable Al in the pH range of 3.5 to 5.0, and oxalic acid did in the range of 3.5 to 4.5.In the citric acid system at pH 4.0, exchangeable Al decreased with the increase in the concentration of citric acid.Whereas the exchangeable Al increased with the increase in the concentration of the organic acid at f irst and then decreased gradually in malic acid system at the same pH.Organic acids can also influence the distribut ion of Al released from kaolinite between kaolinite surface and solution.In the malic acid system, most of Al existed as exchangeable Al on kaolinite surface at lower pH, while most of Al existed as soluble Al in solution at higher pH.In oxalic acid system, most of Al existed as soluble Al in solution and the proport ion of exchangeable Al to total Al released was small in the range of studied pH.The different effects of organic acids on Al release were caused by chemical equilibria in these systems, including dissolution equilibrium of kaolinite, adsorption and desorption of Al, adsorption and desorpt ion of organic acids, dissociation equilibria of organic acids and complexation equilibrium between Al and organic acids.