Both first- and second-order phase transitions have been studied in adsorbed monolayers of n-hexadecyl phosphate at the air-water interface by Brewster angle microscopy (BAM). An initial plateau region in the surface pressure-time (pi -t) curve is caused by a first-order phase transition. This claim is supported by the coexistence of two surface phases which are observed by BAM. A second-order phase transition is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in surface pressure. This unusual transition from an isotropic to an anisotropic phase can be understood considering the intermediate nature of the former phase between liquid-expanded and liquid-condensed states.