The influence of physical diffusion of the electroactive species on the mechanism of charge transport in chemically modified electrodes is analyzed for ion exchange and covalently attached redox polymers with explicit consideration of vacant sites. The appropriate flux equation for charge transport is derived using phenomenological and probability considerations. The influence of both bounded and free physical diffusion rates on the charge transport dynamics is taken into account. The crucial role played by the physical displacement on "observables" such as effective diffusion coefficient, steady-state current, conductivity, transport number, mobility, etc., is demonstrated. The schematic variations of the above parameters on fractional loading and applied potential are shown for bounded and free diffusion conditions. The elucidation of physical displacement mechanism from the experimental response is indicated. The effect of solvent medium on the rate of charge transport is outlined using the formalism.