The present invention is directed to methods for preparation of a composition comprising mixed crystalline polymorphs rifaximin α and rifaximin β. For example, practice of a method of the invention can provide the composition comprising mixed crystalline polymorphs rifaximin α and rifaximin β wherein the rifaximin β is present in about 3-12% (w/w) or is present in about 5-8% (w/w). The composition comprising mixed crystalline polymorphs rifaximin α and rifaximin β is prepared by dissolving raw rifaximin in a water-soluble organic solvent, for example ethanol, at reflux, then adding water to achieve a final mixed solvent of about 7:3 (v/v) solvent to water ratio, then cooling to 35-40° C. until crystallization commences, then further cooling with stirring to 0° C., then recovery of the crystallized material, and drying to a water content of between 2.5% and 5.0%, to provide the composition comprising mixed crystalline polymorphs rifaximin α and rifaximin β. The composition is suitable for medicinal use, such as in treatment of infections of the gastrointestinal tract.
