A comprehensive thermodynamic model for solvent and salt transfer accompanying a partial redox conversion, i.e., conversion between any two oxidation levels, of an electroactive polymer (EAP) film is presented. We discuss two possible cases, namely, one-phase and two-phase behavior of an EAP film. An expression describing the extent of solvent transfer in these situations is presented. Salt transfer is characterized by the difference in permselectivity indices (Delta R-b,R-a) between two oxidation levels of the EAP film. Delta R-b,R-a represents the difference in co-ion (salt) exclusion properties of the EAP in the two different oxidation levels. Delta R-b,R-a is expressed in terms of the EAP's charge, number of electrons transferred in the redox reaction of an electroactive unit, concentration of the supporting electrolyte, salt partition coefficient between solvent and EAP phases, and salt activity coefficients in both phases. Plots of Delta R-b,R-a as a function of the electrolyte concentration allow determining the EAP's phase behavior, ratio of salt partition coefficients, and number of electrons exchanged in the redox process. Delta R-b,R-a is an experimentally accessible quantity; it can be obtained from electrochemical quartz crystal microbalance (EQCM) experiments. Delta R-b,R-a values can be used as a diagnostic tool to characterize an EAP film.