The hydrogen evolution reaction (HER) was followed during the polymerization of aniline on porous platinum electrodes by means of cyclic voltammetry combined with on-line mass spectrometry. It was found that the reaction takes place at the electrode/polymer interface by considering the collection efficiency of the membrane inlet system. It is demonstrated that homogeneous films of polyaniline (PANI) can be deposited onto porous electrode substrates. In this way, a pervaporation membrane is formed with the conducting polymer as the active layer. The permeation of water through a PANI membrane is dependent on the oxidation state of PANI. The higher permeability in the oxidized state is explained in terms of structural alterations during the redox process.