A poly(N-methylaniline) (PNMA) film was obtained by chronoamperometry on tin oxide (TO) in dimethylformamide DMF containing 0.5 M sulphuric acid. The PNMA film formation was monitored by in situ UV–vis spectroscopy. A detailed electropolymerization mechanism and reaction pathway of N-methylaniline (NMA) is proposed based on mass spectroscopy (MS) and NMR analysis. The NMA repeating units (both in the film and in the soluble part) were found to be partially dealkylated during electropolymerization. NMA was also electropolymerized in different solvents such as propylene carbonate (PC) and acetonitrile (ACN). Our results show that protonation is not a necessary condition to initiate the electropolymerization of NMA, but it may play a key role for the propagation of longer conjugated chains. The influence of solvents on the film formation and the function of protons are discussed in this paper.