The ability of the atomic uranium cation U + to activate a variety of saturated and unsaturated hydrocarbon molecules is investigated. Both CH and CC bond activation processes proceeds at markedly higher kinetic efficiencies compared with the lower congener Nd + from the lanthanide series. Formation of a cationic uranium-benzene complex occurs in three consecutive dehydrogenation reactions between U + and ethylene. The mechanism of this metal-mediated cyclotrimerization is enlightened by kinetic measurements, collision-induced dissociation experiments and ion-molecule reactions of the intermediate species.