AbstractNew types of surface‐active organocobaltocenium(I) complexes, η‐CnH2n+1X‐C5H4(ηC5H5)2Co+Y−and(η‐CnH2n+1X‐C5H4)Co+Y−(n= 6–16; X not present, NHCO or OCO; Y = Cl or PF6) were prepared and their surface character studied. (1) The critical micelle concentrations of the cobaltocenium chlorides were much lower than those of corresponding trimethylammonium‐type cationic surfactants. (2) The surface‐active character of the cobaltocenium chlorides in aqueous solution (and the redox potentials of the hexafluorophosphates in acetonitrile) were affected by the substituents (X) in the cyclopentadienyl groups. (3) The surface activities of the cobaltocenium salts were lost on reduction with NaBH4to afford (alkyl‐substituted cyclopentadiene) cyclopentadienylcobalt (0) complexes which were surface‐inactive but could be re‐oxidized to afford the surface‐active cobaltoceium(I) salts. The cobalt complexes mentioned above may be the first examples