The apparent diffusion (permeation) coefficients D-app (referred to the Hittorf system) of HCl in Nafion(R)-117 and polypyrrole-modified Nafion(R)-117 foils which had been subjected to different pretreatments were determined by a new chronopotentiometric procedure. The values of D-app for unmodified foils at 25 degrees C range from 1.4 x 10(-12) to 1.2 x 10(-11) m(2) s(-1) depending on the water uptake. A slight increase in D-app during the experiments is attributed to a probable further water uptake. When polypyrrole (Ppy) was formed inside Nafion(R) using FeCl3 as the oxidizer two effects were observed : one Ppy formation step led to a defined Ppy+Nafion(R) composite with a strongly increasing D during the experiment (from 5.8 x 10(-12) to 1.0 x 10(-11) m(2) s(-1)), whereas after further formation steps an increasing amount of Cl- inside the foil appeared to be due to a subsequent oxidation of already existing Ppy and much lower but constant values of D-app were found (5 x 10(-14) and 2 x 10(-14) m(2) s(-1)).