The capacitance method was applied to the study of the adsorption of anhydrides and imides with different size of the anhydride and imide ring on iron electrodes in acid medium. It was found that the addition of anhydrides and imides to sulphuric acid solution leads to a decrease in the electrode double layer capacitance. The latter is determined by the adsorption of the compounds investigated. Verifying the validity of Frumkin's isotherm adsorption parameters which characterize the interaction forces between the species in the adsorbed layer, their maximum surface excess, as well as the area occupied by a single adsorbate, were determined. The data obtained, and those provided by quantum-chemical calculations of the partial electron charges and the geometric area of the organic molecules studied, form the basis of a discussion on the orientation of the species adsorbed. The inhibiting effect observed is explained in relation to the adsorption behaviour.