The adsorption of N-(phenyl)maleimide and its derivatives on iron electrodes in sulfuric acid medium has been studied by the capacitance method. The process investigated is well described by Frumkin’s isotherm. The adsorption parameters characterizing the interaction forces between the species in the adsorbed layer, the area occupied by each species and the maximum surface excess have been calculated on the basis of this isotherm. Combining data from the electrochemical method with quantum-chemical calculations of the chemical structure and the partial electronic charges of the compounds studied a mechanism of adsorption on the iron surface is proposed. The inhibiting effect observed is explained in relation to the adsorption behaviour.