Mometasone [I; 9a,21-dichloro-1b,17a-dihydroxy-16a-methyl-pregna-1,4-dine-3,20-dione] was prepd. from icomethasone [II; 9a-chloro-11b,17a,21-trihydroxy-16a-methyl-pregna-1,4-diene-3,20-dione] by reaction with a sulfonyl chloride such as methanesulfonyl chloride or p-toluenesulfonyl chloride the presence of a tertiary amine and solvent, and the 21-sulfonate obtained was reacted with a source of chloride ions. Thus, icomethasone 21-sulfonate, obtained by the reaction of methanesulfonyl chloride and II, was reacted with sodium chloride to afford I. I, thus obtained, is a potent anti-inflammatory agent.