The invention relates to the use of a working medium in steam power plants having a steam generator for generating working medium vapor, a pump for generating an increased pressure in the working medium vapor, an expansion machine for generating mechanical and/or electrical work by expansion of the working medium from an elevated pressure to a lower pressure level under working conditions and a condenser for condensing the working medium, wherein the working medium contains a lubricant. The invention is characterized in that the working medium contains acetone and a lubricant.