The invention relates to a method for producing Mannich adducts that contain polyisobutylene phenol by: a) alkylating a phenol with highly-reactive polyisobutylene having a numerical average molecular weight of less than 1 000 and with a polydispersivity of less than 3.0, at a temperature lower than 50 °C and in the presence of an alkylation catalyst; b) reacting the reaction product from a) with; b1) an aldehyde, selected among formaldehydes, with an oligomer and with a polymer of the formaldehyde, and with; b2) at least one amine which has at least one primary or one secondary amino function. The invention also relates to Mannich adducts that can be obtained by using this method, to the use of the Mannich adducts as detergent additives in fuel compositions and lubricant compositions, and to additive concentrates, fuel compositions and lubricant compositions containing these Mannich adducts.