The present invention relates to new salt forms of 5-(2,4-dihydroxy-5-isopropyl-phenyl)-4-(4-morpholin-4-ylmethyl-phenyl)-isoxazole-3-carboxylic acid ethylamide, in particular the mesylate, hydrochloride, tartrate, phosphate and hemi fumarate salt thereof; to crystalline forms of such salts; to polymorphs of 5-(2,4-dihydroxy-5-isopropyl-phenyl)-4-(4-morpholin-4-ylmethyl-phenyl)-isoxazole-3-carboxylic acid ethylamide; to hydrates and polymorphs of the new salt forms mentioned above; to the use of the new salt forms mentioned above for the manufacture of a medicament for the treatment of a disorder mediated by Hsp90; to a method for treating a disorder mediated by Hsp90 using the new salt forms; to formulations comprising such salt forms, in particular aqueous solutions suitable for intravenous administration; and to amber glass containers being filled with such formulations.