A diesel fuel composition comprising a quaternary ammonium salt additive which additive is formed by the reaction of (1) a quaternising agent and (2) a compound formed by the reaction of a hydrocarbyl-substituted acylating agent and at least 1.4 molar equivalents of an amine of formula (B1) or (B2), wherein R2 and R3 are the same or different alkyl, alkenyl or aryl groups having from 1 to 22 carbon atoms; X is a bond or alkylene group having from 1 to 20 carbon atoms; n is from 0 to 20; m is from 1 to 5; and R4 is hydrogen or a C1 to C22 alkyl group.