PURPOSE: To provide 17α-hydroxyprogesterones and corticoids in high purity and high yields by reacting a specified enol steroid with an acid after contact with peroxymonosulfate ions.

CONSTITUTION: The 17α-substd. steroid of formula I (where X is H or no bond, when X is no bond, the C3 dotted and solid double line is a double bond, when X is H, the C3 dotted and solid double line is a single bond; R6 is H, Cl or the like; R9 is H, F or the like; R11 is H, α- or β-hydroxyl or the like; R16 is H, methyl or the like; R17 is H or a group of formula II (where Z is 1-4C alkyl, phenyl or the like); and R20 is 1-5C alkyl, phenyl or the like) or the C3 protected deriv. thereof is brought into contact with peroxymonosulfate ions in a two-phase reactive medium contg. a ketone, preferably acetone in the presence of a phase transfer catalyst, preferably n-tetrabutylammonium hydrogensulfate, and the resultant product is reacted with an acid to obtain the objective compd. of formula III (where R21 is H, 2-6C alkylcarboxylate or the like).