With fluorescence microscopy, we have found that monolayers of dipalmitoylphosphatidylcholine with a small concentration of dye lipids at an air/water interface always contain a number of membrane defects and domains in the liquid expanded phase. These features range in sizes from several micrometers to several millimeters. Furthermore, the fluorescence intensity of these monolayers exhibits a maximum at the full transition from the liquid expanded phase to the liquid condensed phase. At very low surface pressures, monolayers of pure fluorescence dye lipids have a large number of circular defects. It was also found that lipid motion is a common phenomenon with average speeds much larger than the rate of diffusion.