A process for preparing compounds of formula: (II) by esterification of the C-17 hydroxyl group of 6α, 9α-difluoro-11β, 17α-dihydroxy-16α-methyl-3-oxoandrosta-1,4-diene-17β-carbothioic acid, the compound of formula: (I) comprises treating compound (I) with a slight excess of an acyl chloride of general formula R-COCI, where R represents -CH2CH3, -CH2CH2CH3 or -CH(CH3)2, in an inert solvent, in the presence of a tertiary amine. Preferably the process is carried out using pyridine in the presence of acetone at a temperature of from 5 °C to -20 °C. z.