The principles of the on-line electrochemical cell - mass spectrometer connection and an overview of some theoretical and practical aspects of differential electrochemical mass spectrometry (DEMS) are given. DEMS is an experimental technique, which makes it possible to investigate electrosorption, electro-oxidation and electrohydrogenation of substances like aromatic hydrocarbons and other low-molecular-weight organic compounds and reaction intermediates. We have demonstrated the use of our experimental set-up for detection, identification and observation in time of volatile and semivolatile species during electrochemical polarisation of working electrode. The measurements are presented as currents of ions of selected masses of the expected products as a function of working electrode potential. Simultaneously, the Faradaic current - potential curves were measured.