Contact (ΔV) potential measurements have been made for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulphate at the n-heptane/ NaCl solution interface. The results are in good agreement with the predictions based on a recently proposed model of ionized films and may be taken as very strong support for this model. The results also show that the nett dipole change occurring normal to the interface on the adsorption of each ion is constant and independent of the surface coverage.