The diffusion transport of reversible redox species was investigated at microbands in a thin film coating. The effect of the film thickness onto the diffusion was studied theoretically and by numerical simulations both at single-band and double-band microelectrodes. According to the diffusion length of the species in this restricted volume, the corresponding theory and simulations show that several diffusional regimes are expected which provide a way to estimate both the diffusion coefficient and concentration of the species but also to evaluate at the same time the film thickness. This has been demonstrated experimentally by examining the electrochemical responses of microbands coated by Fe-III-loaded Nafion films of different thicknesses. A very good agreement is obtained between theory and experiments which shows that the electrochemical responses of such devices afford a possibility of monitoring the physical and geometrical properties of a polymer film in relation to its chemical environment.