Abstract The electrochemical quartz crystal impedance (EQCI) analysis method was used for the first time to quantitatively examine the precipitation of LiOH onto a gold electrode at potentials negative to ca. −0.7 V vs. SCE during the cathodic sweep reduction of dissolved oxygen and coexisting water in acetonitrile (ACN) containing LiClO 4 · 3H 2 O, as a result of the poor solubility of electrogenerated LiOH in the ACN medium. The suggested LiOH-precipitation mechanism was supported by comparative experiments conducted in ACN containing NaClO 4 · 2H 2 O (or tetrabutyl ammonium chloride/bromide), since large EQCI responses implying a similar precipitation of electrogenerated NaOH were also obtained in the NaClO 4 · 2H 2 O system, but the quartz crystal impedance responses were negligibly small in the two systems of quarternary ammonium salts. The effects of concentrations of LiClO 4 · 3H 2 O and the foreign water added in ACN on the EQCI responses were individually examined, and the maximum frequency shift induced by the LiOH precipitation was found to be as large as about −5 kHz. The cyclic voltammetric growth of polypyrrole (PPY) films at several Au electrodes in fresh ACN solutions of 1 mol L −1 pyrrole + 0.1 mol L −1 LiClO 4 · 3H 2 O were comparatively conducted over three potential-sweep ranges, 0 to 0.85 (A, PPY A ), −1.6 to 0.85 (B, PPY B ) and −2.0 to 0.85 V vs. SCE, respectively, giving that the accompanying precipitation of LiOH notably influenced the polymer growth and porosity. Compared with the normal PPY film (PPY A ), the PPY B after removing the LiOH precipitate formed during the cyclic voltammetric growth of the polymer was more porous, as examined by EQCI and SEM techniques, which resulted in a larger Pt dispersion when Pt particles were electrodeposited on the PPY B /Au electrode in acidic chloroplatinic solution and a higher electrocatalytic activity toward methanol oxidation in aqueous H 2 SO 4 . The proposed protocol of increasing the PPY porosity by introducing removable electrodeposits during PPY’s growth may be of some general interests for other polymers using other removable precipitates.