Electrochemical Quartz Crystal Microbalance (EQCM) was applied to a study of electrosorption of Benzyldimethyldodecylammonium bromide (BDDB) on a gold coated quartz sensor from deaerated 0.1M Na2SO4. The discrete voltammetry and mass curves obtained simultaneously made it possible to compute the effective equivalent molar mass (EMM) of the reaction species (the ratio of effective molecular weight of the species to the number of electrons exchanged) as a function of the potential applied. It is found that a cathodic mass increase of gold electrode in blank 0.1M Na2SO4 solutions can be accounted for hydrogen evolution and the apparent quartz sensor response due to the change of the electrode-solution mechanical coupling. This effect completely disappears in the presence of a cationic surfactant as BDDB which increases the hydrogen evolution overvoltage. The low concentrations of BDDB as 10(-5) M are found to inhibit anodic oxide formation on gold. At the concentrations higher than 10(-4) M oxidation and reduction processes appear at 1.04 V and 0.69 M nhe respectively. The oxidation is accompanied by intensive electrosorption of organic species which are to a high extent desorbed upon the reduction. Although at higher concentrations, ca. 10% of the adsorbate remains on the surface upon the reduction. The EMM values obtained on the basis of the voltammetric cycles and for BDDB electrosorption at a constant potential give a clue to the analysis of the chemical transformations in the system. The process of electrosorption of dimethylbenzalkonium ion appears to be connected with the adsorption of the Pr anions from the solution and the formation of the anodically adsorbed oxygen layer. The EMM values obtained for the electrosorption in stationary conditions was 260 g/mole. The ir and XPS spectroscopy data indicate that oxidation of BDDB results in a shrinkage of the alkyl chain and formation of amide and quinonic C=O bonds, which conforms with the obtained EMM value, assuming one electron exchange.