A recently developed gel-sol method for the synthesis of monodispersed particles was applied in the preparation of uniform basic aluminum sulfate (Al-3(SO4)(2)(OH)(5) . 2H(2)O) microcrystals of different shapes including round-cornered cubes, sharp-edged cubes, truncated-corner cubes, hexagonal platelets, and ellipsoids. The process involves aging of a highly condensed aluminum hydroxide gel in the presence of sulfate ions under acidic conditions at 100 degrees C for 3 days or longer. The growth of the basic aluminum sulfate particles was found to proceed through a dissolution-recrystallization mechanism from the aluminum hydroxide gel. The morphological variations were explained in terms of the specific adsorption of anions, such as SO4-2, OH-, and Cl-, to different crystal planes during the growth. It has been shown that the technique of the so-called supersaturation quenching enhances the uniformity of the sharp-edged cubic particles and that the systematic control of the mean size is possible by seeding.