Cobaltocenium-labeled polymers were prepared by atom transfer radical polymerization (ATRP) using cobaltocenium-containing alpha-haloesters as initiators. The locations of the cobaltocenium moiety in the polymer chains (either at the end or in the middle) were dictated by the chemical structures of initiators. Kinetic studies showed that polymerizations of styrene, tert-butyl acrylate, and methyl methacrylate monomers followed a controlled/living manner, except that polymerization of methyl methacrylate with the aid of cobaltocenium monoinitiator displayed significant termination. All resultant polymers were redox-active and exhibited characteristic UV-vis absorption from the cobaltocenium moiety.