The use of betaine compounds of the formula R1—CO—NH—X—NR2R3)+—Y—COO− where R1 denotes a linear or branched alkyl or alkenyl radical having 5 to 21 carbon atoms, R2 and R3 each independently denote C1- to C4-alkyl radicals, X denotes a hydrocarbon bridging element having 1 to 12 carbon atoms and Y denotes a linear or branched C1- to C4-alkylene group, as additives for fuels, especially as detergent additives for diesel fuels.