Additive composition for use in a fuel or lubricant formulation, comprising an active substance in an inclusion complex with a modified cyclodextrin of formula (I): wherein n is an integer from 6 to 20, and R1, R2 and R3 are each independently selected from hydrogen, optionally substituted alkyl, optionally substituted aryl and carbonyl, provided that R1, R2 and R3 are not all hydrogen. Also provided is a fuel or lubricant formulation comprising the additive composition, a premix for use in preparing the additive composition, and the use of a modified cyclodextrin (I) as a vehicle for an active substance in an additive composition or in a fuel or lubricant formulation.