Monolayer properties such as phase behavior, collapse pressure, and surface viscosity are determined by monolayer composition. Learning how to control these properties through simple additives is important to understanding lung surfactant monolayers and to designing optimal replacement surfactants for treatment of disease. The properties of Infasurf, a replacement lung surfactant derived from calf lung lavage organic extract, can be modified in a controlled fashion by adding hexadecanol (HD). Grazing incidence X-ray diffraction shows that the HD preferentially interacts with dipalmitoylphosphatidylcholine (DPPC), the main phospholipid component of Infasurf, in the same way as in binary mixtures of DPPC and HD. HD intercalates between the DPPC chains, which leads to a tighter packing of the two-dimensional lattice and greater stability of the solid phase. This molecular reorganization triggers changes at the macroscopic scale, leading to a greatly increased solid-phase fraction at a given surface pressure and order of magnitude increases in the surface shear viscosity. However, the collapse pressure decreases, and, hence, the minimum surface tension of the film increases.