Monolayers of distearoyl phosphatidylcholine (DSPC) dimyristoyl phosphatidylcholine (DMPC), and mixtures of these phospholipids, and distearoyl phosphatidylcholine (DPPC), were observed at the air-water interface with Brewster angle microscopy. Their properties were compared to pi -A isotherms. The length of the alkyl chain of phosphatidylcholine affected the morphological properties. At low surface pressure DSPC and DPPC, formed the condensed domain, however, DMPC did not form a condensed domain and displayed a larger limiting area per molecule than DSPC. When DSPC and DMPC were mixed together at the same concentration, them property of the spread monolayer in a Tr-A isotherm was more likely to be DMPC however, the BAM images were different from that of DMPC or DSPC at various surface pressures. The phase in pi -A isotherms was not necessarily the same as the BAM images. Even when surface pressure increased with compression, a small collapse was observed in a microscopic region by BAM.