The adsorption of a quaternary ammonium cation from a 3-trimethyl[4-(1-oxo-2-phenylinden-3-yl)-phenyl]ammonium iodide on polycrystalline iron in acidic and neutral media is studied by electrochemical techniques. Assuming the validity of the second-order approach of the biparallel capacitor model, the dependence of the electrode charge on potential is evaluated for the maximum additive concentration using capacitance vs potential data as a starting point. According to the model predictions, a nonlinear least-squares fit of this dependence provides estimates for the adsorption parameters. The orientation of the adsorbed species on the electrode surface is discussed. From dc polarization and ac impedance data, the inhibition efficiency of trimethylamino-2-phenyl-indenonylammonium iodide with respect to iron corrosion in 1 M H2SO4 is evaluated.