New extractants are proposed for the highly effective extraction and concentration of sulphoxides from hydrocarbon feedstock, consisting of acetonitrile solutions of sulphuric acid. The use of these extractants makes it possible to increase considerably the effectiveness of sulphoxide extraction by comparison with polar organic solvents and the traditionally used aqueous solutions of sulphuric acid, and also to concentrate sulphoxides by a factor of 5-10. The procedures developed for sulphoxide extraction have made it possible for the first time to isolate (in pure form) and investigate sulphoxides of heavy vacuum gas oil.