R-α-Lipoic acid and its homologs are converted to their magnesium salts in the presence of a reduced form of the acid, dihydro-(+)-lipoic acid in the case of R-α-lipoic acid itself. The reduced form serves as a polymerization inhibitor, resulting in a Mg di-R-α-lipoate product of higher purity. The reduced acid retained in the product tends to convert to the starting acid, thereby avoiding the inclusion of an extraneous polymerization inhibitor in the product.