Metallocene-containing polyelectrolytes show many unique properties in a variety of electrochemical, optoelectronic, medicinal, and magnetic applications. The utilization of counterions plays crucial roles in many aspects of these polyelectrolytes. This paper reports the first quantitative analysis of counterion exchange in metallocene-containing polyelectrolytes. Particularly counterion exchange of cationic cobaltocenium-containing polyelectrolytes was analyzed by diffusion NMR, which determined molar fractions of dissociated and associated ions as well as ion-exchange constant. Qualitative impact of counterion exchange on macromolecular conformation was directly observed from cobaltocenium-containing molecular brushes. This study may provide valuable guidance on applications such as layer-by-layer assembly and ion-triggered drug delivery that are involved with ion exchange of polyelectrolytes.