State-of-the-art ac impedance models of redox-polymer films attached to an electrode and immersed in a bathing electrolyte usually do not predict the transition region correctly between high frequency Warburg behavior and low frequency capacitive behavior. Two effects are considered here that can give rise to distortions in this spectral region. The first occurs when transport of charge-compensating counterions in the bathing electrolyte is slow relative to charge transport within the polymer film. The second occurs when the system has a nonuniform primary-current distribution (e.g., the disk geometry), and the ohmic resistance of the bathing electrolyte is large relative to the transport resistance of the polymer coating. Both these situations are considered using mathematical models. Characteristic parameters are identified which are convenient for diagnosing when the effects are expected to be observed. Analysis of representative data in the literature shows that neither phenomenon is the cause of the majority of the transition-region nonidealities observed in published ac impedance spectra, although the second effect is important in several studies. The need persists to verify previously postulated causes, such as film-thickness nonuniformities, or to identify and verify others.