An electroactive label was attached to poly(acrylic acid) (molecular weight = 5000) by reaction with ferrocenylethanol. The resulting ferrocene label was stable when the polymer was dissolved in aqueous solutions, and the diffusion coefficient of the labeled polymer was estimated from the currents obtained as the ferrocene was oxidized at a rotating disk electrode. Although strong adsorption of the polymer on the electrode surfaces depressed the oxidation currents below the convection-diffusion controlled values at rotating disk electrodes, it was still possible to estimate diffusion coefficients by appropriate analysis of the rotation rate dependences of the measured currents. At pH values where the labeled polymer became a polyanion, its diffusion coefficient was also estimated from the reduction currents for Ru(NH3)(6)(3+) or Co(NH3)(6)(3+) counterions bound electrostatically to the polyelectrolyte. Good agreement was obtained between the diffusion coefficients estimated by the two independent routes.