The outer membrane (OM) of the Gram-negative bacterial cell envelope is an important biological interface as it initiates bacterial cell adhesion, activates innate immunity during infection, withstands environmental stresses and acts as a gate keeper to nutrients and toxins. The OM is a complex structure consisting of an asymmetric bilayer with an inner leaflet of phospholipids and an outer leaflet of lipopolysaccharide (LPS) with LPS consisting of approximately 75 % of the Gram-negative bacterial surface. Due to the complexity of LPS there are very few OM models that include this important molecule and thus little is known about LPS structure at interfaces. In this work we used a truncated LPS from E. coli (Rc-LPS, consisting of Lipid A and the first seven sugars of the core polysaccharide) to form stable monolayers at the air – liquid interface. Neutron and X-ray reflectometry were used to probe the vertical structure of the monolayers, while the lateral structure was studied with grazing incidence X-ray diffraction and Brewster angle microscopy. It was found that at surface pressures above 20 mN/m the RcLPS monolayers could be resolved as hydrocarbon tails, inner headgroup, and outer headgroup of polysaccharide with increasing solvation from tails to outer headgroup. The lateral organisation of the hydrocarbon tails was of an oblique hexagonal unit cell across all surface pressures with only the tilt angle of the chains changing with surface pressure . The study of the monolayer structure provides the first complete analysis of a realistic E. coli OM surface model.