The microscopic equilibrium structures of adsorbed films of quaternary ammonium surfactants on mica have been investigated by noncontact atomic force microscopy imaging as a function of alkyl chain length and headgroup structure. Spherical and cylindrical surface micelles were observed; these were found to be related to bulk. solution self-assembly and the surfactant packing parameter, v/a(0)l(c). Shape transitions in the surface aggregates were observed on changing the counterion between chloride, bromide, and salicylate.