The redox-active surfactant (11-ferrocenylundecyl)trimethylammonium bromide (Fc(CH2N+(CH3Br-; Fc = [eta(5)-C5H5]Fe[eta(5)-C5H5]), in combination with electrochemical methods, permits reversible control over the surface tension of an aqueous solution. The change in surface activity of the surfactant was caused by oxidation of the electrically neutral ferrocenyl moiety to the polar ferrocenium cation. Oxidation of 0.1 mM Fc(CH2N+(CH3)(3) dissolved in aqueous 0.1 M Li2SO4 caused the surface tension of the solution to increase from 49 to 72 mN/m. The change in surface tension was reversible; reduction of the dicationic Fc(+)(CH2N+(CH3)(3) to Fc(CH2N+(CH3)(3) recovered the initial surface tension of the solution (49 mN/m). Surface tensions of aqueous solutions were cycled repeatedly (> 14 cycles) between these limits. Surprisingly, oxidation of Fc(CH3)(3) at concentrations greater that 10 mM caused a decrease in the equilibrium surface tension. The limiting area per molecule at the surface of the aqueous solution (0.1 M Li2SO4, pH 2) was unexpectedly similar for both Fc(CH2N+(CH3)(3) (85 +/- 4 Angstrom(2)) and Fc(+)(CH2N+(CH3)(3) (75 +/- 4 Angstrom(2)).