Abstract
The system 
was investigated by the ac‐impedance method, covering both the semi‐infinite and the finite (very low frequency) diffusion regimes. Upon decreasing the modulation frequency, a transition from semi‐infinite diffusion to finite diffusion is observed, with different apparent diffusion coefficients calculated for the two limiting cases. The results can be accounted for by a distribution of diffusion coefficients in the film and are quantitatively consistent with a model that assumes two parallel diffusion paths in the film. The values obtained by curve‐fitting to experimental results are 
, and 
, with comparable concentrations of reactive ions in the two forms. The two parallel paths are attributed to different domains of the polymer.