Sterically hindered 17α-hydroxyl groups in steroids are silylated by trimethylsilylimidazole (TSIM) at 100° C. Decreasing rates of reaction were found for the following structures: 17α, 20β, 21-triol > 17α, 20α-diol > 17α, 20α, 21-triol. When methoxime derivatives are formed as the first step, HCL present in the excess of reagent catalyses the silylation reaction of hydroxyl groups. A simple procedure is described whereby methoxime (MO) derivatives are formed in 15 minutes at 60° C (the 11-one group does not react under these conditions) and persilylated compounds are then prepared in 2 hours at 100° C by reaction with trimethylsilylimidazole.