Well‐defined and reproducible reduction waves of undiluted acetonitrile have been obtained at Pt microelectrodes in the presence of a minimum of 5×10−3 M tetraalkilammonium perchlorates (TAAP). At smaller TAAP concentrations, traces of alkali metal ions interfered and the wave plateau could not be obtained. Neither was it possible to obtain a total oxidation wave of acetonitrile in the presence of a small concentration of LiClO4, in this case due to the oxidation of the ClO4− counterions. An addition of water to acetonitrile led to the formation of a new cathodic wave, the height of which was proportional to concentration of water up to 0.022 mol/L. The detection limit of determination of water in pure acetonitrile was estimated as 0.0090 mol/L.