Monolayer behavior of a poly(ethylene glycol)-grafted distearoylphosphatidylethanolamine with PEG molecular weight 5000 (DSPE-PEG5000) was studied by a variety of experimental methods. Slope changes in the surface pressure-molecular area (pi-A) isotherm suggest two transitions in the DSPE-PEG5000 monolayer upon compression which were previously attributed to conformational changes in the polymeric moiety. However, comparative analysis of our experimental data implies that the second slope change in the isotherm at pi approximate to 18 mN/m originates from the monolayer collapse. As visualized by Brewster angle and atomic force (AFM) microscopy, the fluid homogeneous DSPE-PEG5000 monolayer is unable to sustain high pressures and collapses through vesicular structures formed as a result of 2D to 3D relaxation of an overcompressed film. AFM topographic images provided a height value of 9 nm for the collapsed structures on top of the monolayer, which can be referred to as the height of DSPE-PEG5000 lamellar structures. Thus, the collapse at a surprisingly low surface pressure implies that a distinct region where the PEG5000 moiety would adopt a brush conformation is probably never achieved for the DSPE-PEG5000 monolayer. Ellipsometric measurements of monolayer thickness confirm that although an elongated conformation is developed prior to the monolayer collapse, this conformation is better described as an "extended mushroom" rather than a "brush".