The structure of aggregates of lithium perfluor ooctanesulfonate (LiFOS) adsorbed to the interface between graphite and aqueous solution have been measured. This fluorocarbon surfactant produces aggregates which are long (similar to 100 nm) and thin (similar to 5 nm), and about one molecule (similar to 1.3 nm) deep. The aggregates lie in straight, parallel arrays on the surface with a characteristic repeat di stance, or period, perpendicular to the long axis, As the bulk concentration of LiCl is increased, the period decreases, but as the bulk concentration of LiFOS is increased, the period increases. The decrease in period on addition of salt is similar to that observed for sodium dodecyl sulfate (SDS) and is explicable in terms of electrostatic forces. The increase in period on addition of surfactant is in sharp contrast to the behavior of SDS and may be due to a higher surfactant packing-parameter for LiFOS.