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—COCl, 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.