In this paper, the general theory of coupled electron-ion transport inside a film having a mixed electron-ion coductivity is applied, and the a.c. impedance expressions have been calculated for the situations where the film can exchange electrons and/or ions with the surrounding media. Important example of such system is given by a conducting polymer film between two solutions containing a redox active couple ("redox active solutions") which is subjected to a faradaic process at the film surface with the participation of electronic species inside the film. As particular cases, the general formula describes also the systems where one or both interfacial boundaries can only be crossed by one type of charged species, electrons or ions. Besides the previously analysed situations, metal/film/ metal, metal/film/background solution, or a film between two background solutions, we have obtained expressions for impedance of the film between a redox active solution and a background (without redox species) one, or between a metal and a redox active solution. Theoretical predictions for the shapes of complex impedance plots in the case of a redox active solution have been demonstrated.