Potential control of soluble aluminum in drinking water by formation of solids other than Al(OH)3 was examined. At pHs below 6.0, Al(+3) solids containing sulfate, silica or potassium are thermodynamically favored versus amorphous Al(OH)3; however, in this work no evidence could be obtained that solids other than Al(OH)3 would form in practice. At pHs above 9, aluminum and magnesium were discovered to form complex solid phases of approximate composition AlMg2(OH)7, AlMg2SiO2(OH)7 or Al(SiO2)2(OH)3 dependent on circumstance. Formation of these solids provide a mechanistic explanation for enhancements to precipitative softening obtained in practice by dosing Al(+3) salts; that is, improved flocculation/settling and removal of silica from water that interferes with calcium precipitation. The solids also maintain residual aluminum below regulatory guidelines at high pH > 9.5.