Polyisobutenamines of the formula I: R--X (I) where R is a polyisobutenyl radical having a number average molecular weight M.sub.n of from 500 to 1500 and a polydispersity M.sub.w/ M.sub.n of less than 1.4 and X is an organic radical having amino groups, are prepared and are used as detergent additives in fuel compositions. Additive concentrates contain at least one polyisobutenamine in an amount of from 0.1 to 80% by weight.