An effective method of dehydrating 9_-hydroxyandrost-4-ene-3,17-dione by mineral acids in organic solvents, producing essentially quantitative formation of androsta-4,9(11)-diene-3,17-dione is proposed. Quantitative isomerization of the side product androsta-4,8(9)-diene-3,17-dione to target product was shown to be pssible.