Control of aluminum (Al) solubility in water by solids other than Al(OH)3 was explicitly tested at pHs below 6.0 and above 8.5. At lower pHs the presence of sulfate promoted formation of lower Ksp Al(OH)3 solid phases at pH 4.7-5.6; however, the presence of sulfate, silica or potassium did not cause formation of different solid phases. At higher pHs, aluminum andmagnesium were discovered to form a discrete solid phase in the pH range of 9.0-10.5 with the approximate formula Al2Mg(OH)7. When silica was also present in these systems, a different solid formed with the approximate formula Al2MgSiO2(OH)7, which gradually transformed into still another solid phase of approximate formula Al(SiO2)2(OH)3. Formation of these solids has direct implications for enhanced softening by dosing of alum coagulants and in control ofeffluent aluminum residuals. Includes 13 references, tables, figures.