In situ neutron reflectivity measurements have been made on spin cast polyvinylferrocene (PVF) films maintained under electrochemical potential control. The data show that the interiors of both oxidized and reduced films are close to homogeneous. The results show that neutron reflectivity measurements, including isotopic substitution, can provide novel insights into film composition and structure at the electrode/polymer/solution interface. Isotopic substitution of the solvent has allowed the contributions of the solvent and the solute to the total film composition to be separated. Responses were determined to variations in the imposed potential (a constraint at the electrode/polymer interface) and the electrolyte concentration (a constraint at the polymer/solution interface). Solvent entry occurs upon oxidation, resulting is a more diffuse polymer/solution interface. Only at very high concentrations is salt permeation into PVF films sufficiently high to be considered as a possible source of co-ions for redox-driven transfer in order to satisfy electroneutrality.