A diesel fuel composition comprising a first additive (i) comprising a quaternary ammonium salt and a second additive (ii) comprising a Mannich reaction product; wherein the quaternary ammonium salt additive (i) is formed by the reaction of a compound of formula (A): R O O R1 10 (A) and a compound formed by the reaction of a hydrocarbyl-substituted acylating agent and an amine of formula (B1) or (B2): 15 N X R3 R2 NHR4 N X R3 R2 [O(CH2)m]nOH (B1) (B2) wherein R is an optionally substituted alkyl, alkenyl, aryl or alkylaryl group; R1 is a C1 to C22 alkyl, aryl or alkylaryl group; 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 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; and wherein the Mannich reaction product additive (ii) is the product of a Mannich reaction between: (d) an aldehyde; (e) an amine; and 25 (f) a substituted phenol; wherein the phenol is substituted with at least one branched hydrocarbyl group having a molecular weight of between 200 and 3000.