An additive composition for use in a diesel fuel formulation, comprising a cetane improver 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 diesel fuel formulation comprising the additive composition, and the use of a modified cyclodextrin (I) as a vehicle for a cetane improver in an additive composition or diesel fuel formulation.