Dipalmitoylphosphatidylcholine (DPPC) is the major component of lung surfactant, which stabilizes the lung by reducing greatly the surface tension at the air/liquid interface of the alveoli. In this study, the adsorption behavior of DPPC at the air/water interface was investigated with tensiometry, infrared reflection-absorption spectroscopy (IRRAS), and ellipsometry. Two different preparation protocols were used to vary the size and microstructure of dispersed particles, and their effects were assessed. The tension results indicate that sonication of DPPC above the main gel-to-liquid-crystal transition temperature, to break the large liposomes into smaller vesicles, greatly improved the adsorption rate and tension-reduction ability of DPPC dispersions. In IRRAS, the intensity of the v(a)-CH2 band was found to be much higher for DPPC dispersions than the value for spread DPPC monolayers at the maximum monolayer surface density, suggesting that after surface film formation there may be substantially more material than a monolayer associated with the air/water interface. The extra material detected by IRRAS is probably due to some DPPC vesicles or liposomes close to the interface or attached to the monolayer. Although the extra material does not contribute to the ellipsometry or the surface tension at constant area, it influences the dynamic tension under area oscillation. These results are consistent with the idea that formation of a surface film from DPPC dispersions involves two steps: diffusion of particles (vesicles or liposomes) from the bulk phase, followed by their partial disintegration to form an insoluble surface film and a surface associated reservoir.