Lung surfactant (LS) is a unique mixture of lipids and proteins that lines the alveoli and lowers the surface tension in the lungs, thereby insuring a negligible work of breathing. The adsorption of LS to the alveolar air-liquid interface is strongly inhibited by the competitive adsorption of surface active serum proteins, and is likely the explanation of surfactant inactivation in Acute Respiratory Distress Syndrome (ARDS). In vitro, the adsorption of clinical replacement LS to the interface is restored by the addition of hydrophilic non-adsorbing polymers such as polyethylene glycol (PEG) suggesting a promising therapy for ARDS. Utilizing in situ grazing incidence x-ray diffraction (GIXD) and x-ray reflectivity (XR), we have examined the surface ordering of clinical replacement LS at the air-liquid interface and the effect of albumin, a model serum protein, and PEG. XR measurements confirm that serum proteins impose a steric barrier to LS adsorption, inhibiting the LS characteristic GIXD peaks. In the LS free system, scattering experiments show no evidence of PEG surface ordering while LS on a PEG subphase shows only a subtle condensation of the LS laterally. However, the addition of PEG to albumin inhibited LS restores the LS characteristic XR and GIXD peaks and progressive cycling shows the LS replacing the albumin on the interface. These scattering results are consistent with fluorescence images of the interface which show a coexistence of ~1000 μm albumin and LS regions until all albumin regions are eventually expelled from the interface. The results are also consistent with recent work showing a PEG induced depletion attraction between the LS aggregates and the interface is responsible for the increased LS adsorption in the presence of PEG.