The biosynthetic intermediates of bile acid, 3α,7α,12α-trihydroxy- and 3α,7α-dihydroxy-5β-cholestan-26-oic acids, were synthesized by means of the thermal elimination of β-ketosulfoxides. The α, β-unsaturated ketones as key compounds of the synthesis, 3α,7α,12α-trihydroxy- and 3α,7α-dihydroxy-5β-cholest-25-en-24-ones, were effectively derived from the β-ketosulfoxides prepared from methyl cholate or chenodeoxycholate by reaction with methylsulfinylcarbanion. These unsaturated ketones were converted into 3α,7α,12α,26-tetrahydroxy- and 3α,7α,26-trihydroxy-5β-cholestanes by reductive deoxygenation and hydroboration, of which stereoisomers were chromatographically separated into 25S- and 25R- isomers. The oxidation of each of the above isomeric alcohols after the protection of the hydroxyl groups on the steroidal ring and the following hydrolysis gave the title 26-carboxylic acids.