(6α,11β,16α,17α)-6,9-Difluoro-17-[(2-furanylcarbonyl)oxy]-11-hydroxy-16-methyl-3-oxoandrosta-1,4-diene-17-carbothioic Acid is a steroid used in the preparation of crystalline forms of the therapeutically useful anti-inflammatory corticosteroid fluticasone furoate (I) and its solvates. A fluticasone furoate (F599510) impurity.