Abstract: A series of polyaluminum chloride sulfate ion (PACS), which have different basicities and Al ³⁺/SO²₄^- molar ratio, were prepared by using AlCl₃·6H₂O, Na₂SO₄ and NaOH as raw materials. The effects of Al³⁺ /SO²₄^- molar ratio and basicities on the aluminum species of PACS were investigated by using Al-Ferron timed complex colorimetric method. A series of experimental results were achieved. Nonlinear formulae modelling the aluminum species of PACS, Al³⁺ /SO²₄^- molar ratio and basicities were presented.