The use of a reaction product comprising a quaternized nitrogen compound, or of a fraction thereof which comprises a quaternized nitrogen compound and is obtained from the reaction product by purification, wherein the reaction product is obtainable by reacting a quaternizable alkylamine comprising at least one quaternizable tertiary amino group with a quaternizing agent which converts the at least one tertiary amino group to a quaternary ammonium group, wherein the quaternizing agent is a compound of the general formula 1 R 1 OC R 2 in which R 1 is a saturated, straight-chain or branched hydrocarbyl radical having 1 to 4 carbon atoms and R 2 is an optionally substituted monocyclic aryl or cycloalkyl radical, where the substituent is selected from OH, NH 2, NO 2, C OR 3, and R 1 OC -; in which R 2 is as defined above and R 3 is H or R 1; or R 2 is R 1a OC - in which R 1a has the definitions given above for R 1; or wherein the quaternizing agent is a compound of the general formula 2 R 1 OC -A-C OR 1a in which R 1 and R 1a are each independently a saturated, straight-chain or branched hydrocarbyl radical having 1 to 4 carbon atoms and A is an optionally mono- or polysubstituted, straight-chain or singly or multiply branched hydrocarbylene bridging group which has 1-10 carbon atoms and may optionally be interrupted by one or more heteroatom groups; or wherein the quaternizing agent is an epoxide of the general formula 4 where the R d radicals present therein are the same or different and are each H or an aliphatic or aromatic radical having at least 1 to 10 carbon atoms, where the aliphatic radicals R d may additionally contain heteroatoms, such as O, N, NH, S, in their chain; alone or in combination with a free acid; or wherein the quaternizing agent is a dialkyl carbonate; and wherein the alkylamine comprises at least one compound of the following general formula 3 R a R b R c N in which at least one of the R a, R b and R c radicals is a straight-chain or branched, saturated or unsaturated C 8 -C 40 -hydrocarbyl radical which does not have any heteroatoms and the remaining radicals are identical or different, straight-chain or branched, saturated or unsaturated C 1 -C 6 -hydrocarbyl radicals; or in which all R a, R b and R c radicals are identical or different, straight-chain or branched, saturated or unsaturated C 8 -C 40 -hydrocarbyl radicals which do not have any heteroatoms; as a diesel fuel additive for reduction and/or prevention of internal diesel injector deposits (IDIDs) in direct injection diesel engines. reacting a quaternizable alkylamine comprising at least one quaternizable tertiary amino group with a quaternizing agent which converts the at least one tertiary amino group to a quaternary ammonium group, wherein the quaternizing agent is a compound of the general formula 1 R 1 OC R 2 in which R 1 is a saturated, straight-chain or branched hydrocarbyl radical having 1 to 4 carbon atoms and R 2 is an optionally substituted monocyclic aryl or cycloalkyl radical, where the substituent is selected from OH, NH 2, NO 2, C OR 3, and R 1 OC -; in which R 2 is as defined above and R 3 is H or R 1; or R 2 is R 1a OC - in which R 1a has the definitions given above for R 1; or wherein the quaternizing agent is a compound of the general formula 2 R 1 OC -A-C OR 1a in which R 1 and R 1a are each independently a saturated, straight-chain or branched hydrocarbyl radical having 1 to 4 carbon atoms and A is an optionally mono- or polysubstituted, straight-chain or singly or multiply branched hydrocarbylene bridging group which has 1-10 carbon atoms and may optionally be interrupted by one or more heteroatom groups; or wherein the quaternizing agent is an epoxide of the general formula 4 where the R d radicals present therein are the same or different and are each H or an aliphatic or aromatic radical having at least 1 to 10 carbon atoms, where the aliphatic radicals R d may additionally contain heteroatoms, such as O, N, NH, S, in their chain; alone or in combination with a free acid; or wherein the quaternizing agent is a dialkyl carbonate; and wherein the alkylamine comprises at least one compound of the following general formula 3 R a R b R c N in which at least one of the R a, R b and R c radicals is a straight-chain or branched, saturated or unsaturated C 8 -C 40 -hydrocarbyl radical which does not have any heteroatoms and the remaining radicals are identical or different, straight-chain or branched, saturated or unsaturated C 1 -C 6 -hydrocarbyl radicals; or in which all R a, R b and R c radicals are identical or different, straight-chain or branched, saturated or unsaturated C 8 -C 40 -hydrocarbyl radicals which do not have any heteroatoms; as a diesel fuel additive for reduction and/or prevention of internal diesel injector deposits (IDIDs) in direct injection diesel engines.