Electropolymerization, morphology characterization, and ion transport of poly(3,4-ethylenedioxythiophene) (PEDOT) films doped with different counterions ( chloride, ferrocyanide (FCN), and poly(p-styrenesulfonate) (PSS-)) on a platinum electrode were investigated using scanning electrochemical microscopy (SECM) during both potential step (chronocoulometric) and cyclic voltammetric scans. An ultramicroelectrode (UME) tip was positioned close to the surface of a PEDOT-modified substrate electrode, and the responses of both electrodes to a substrate potential step or linear sweep were monitored simultaneously. Chloride or ferrocyanide ( FCN) ejection during PEDOT reduction was shown to be a function of the reduction potential. The nature of the cation in the bulk solution was not found to be important in the kinetics of ion transport in PEDOT+/FCN- films. Direct evidence for the incorporation of cations of Ru(NH3)(6)(3+/2+) in a PEDOT film during its reduction was also obtained by SECM measurements. The adsorption of Ru(NH3)(6)(3+) in fully oxidized PEDOT+/PSS- films was observed and attributed to ion exchange between the Na+ co-ion of PSS-and Ru(NH3)(6)(3+) in the bulk solution.