It has been shown by i.r. reflection spectroscopy that a deposit of benzyltrimethyl-ammonium hydroxide is formed at the cathode during the electrochemical, reduction of the benzyltrimethylammonium cation in HMPA; this deposition has been studied in situ by polaromicrotribometry. The deposit is accompanied by the formation of o-methylbenzyldimethylamine. The very hygroscopic benzyltrimethylammonium hydroxide probably complexes residual water at the electrode and thus favours a Sommelet-Hauser rearrangement.