Interfacial properties of zwitterionic surfactants having polymethylene intercharge arms of different length have been studied. Critical micelle concentration values, area per molecule, and surface tension values are all consistent with the following picture of the surfactant film: molecular interactions inside the interfacial film are electrostatic dipole-dipole and steric in origin. Steric interactions are predominant for large and flexible intercharge arm molecules for which the hydrophobic polymethylene arm folds as a loop inside the hydrophobic part of the interface. Physical properties of the surfactant film can then be controlled by the molecular properties of the surfactant, mainly the dipole moment of the zwitterionic headgroup and the flexibility of the intercharge arm.Among water-soluble surfactants, zwitterionic compounds have been recognized as especially stable against external variations, particularly ionic strength and temperature.*