Four different phases and four different first-order phase transitions have been shown to exist in Gibbs adsorption layers of mixtures containing n-hexadecyl dihydrogen phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2. These conclusions have been made from surface pressure-time (pi-t) adsorption isotherms measured with a film balance and from monolayer morphology observed with a Brewster angle microscopy (BAM). The observed four phases are gas (G), liquid expanded (LE), liquid condensed (LC) and LC' phases. Three first-order phase transitions are G-LE, LE-LC and LC-LC'. However, the thermodynamically allowed G-LC phase transition in a 1.2 x 10(-4) M mixture at 2 degrees C, which is below the so-called triple point, is kinetically separated into the G-LE and LE-LC phase transitions. The most interesting observation is that the homogeneous LC phase shows a new first-order phase transition named as LC-LC' at 2 or 5 degrees C. The LE and LC phases represent circular and fractal shaped domains, respectively, whereas the W phase shows very bright, anisotropic and characteristic shaped domains. (c) 2006 Elsevier Inc. All rights reserved.