The adsorption of dimethyl sulfoxide (DMSO) at the mercury/DMSO + 0.15 M Na2SO4 interface is studied using the electrode charge density as the independent electrical variable and taking into account medium effects. In order to perform this analysis the mean activity coefficients of the salt in the presence of DMSO were experimentally determined and the electrical double layer parameters calculated by using Fawcett’s function. The results suggest that upon adsorption the DMSO molecule, in the concentration range used, may suffer a partial reorientation process. The system can be described by Frumkin’s isotherm but it is necessary to consider a variation of the maximum surface excess with the electrode charge.