Polyisobutenylphenol-containing Mannich adducts are prepared by a) alkylation of a phenol with highly reactive polyisobutene at below about 50° C. in the presence of an alkylation catalyst; b) reaction of the reaction product from a) with formaldehyde, an oligomer or a polymer of formaldehyde and at least one amine which has at least one secondary amino function and no primary amino function, or c) reaction of the reaction product from a) with at least one adduct of at least one amine which has at least one secondary or primary amino function and formaldehyde, an oligomer of formaldehyde, a polymer of formaldehyde or a formaldehyde equivalent, and are used as detergent additives in fuel and lubricant compositions, and additive concentrates, fuel compositions and lubricant compositions contain these Mannich adducts.