An improved detergent for use in hydrocarbon fuels comprising the composition produced by mixing (1) the Mannich reaction product of a polyisobutene-substituted phenol, (preferably, where the polyisobutene from which the substituted phenol is derived contains at least 70% terminal olefinic double bonds of the vinylidene type); an aldehyde; and an amine, preferably, ethylene diamine; and (2) a compound selected from the group consisting of an alkylbenzene sulfonic acid, an alkylnapthalene sulfonic acid, acetylaetone and mixtures thereof. Preferably, the alkylbenzene and alkylnapthalene sulfonic acid is selected to be a (C6 to C18) alkylbenzene sulfonic acid/napthalene sulfonic acid, especially preferred being dodecylbenzene sulfonic acid.