We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate n-TDP) at the air-water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure (70-time (t) adsorption isotherm is found to Occur from 2 to 20 degrees C A with appropriate concentrations of the aniphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the Surface pressure is caused by I first-order phase transition. Only this phase transition is observed at < 6.5 degrees C and it is considered I,, it gas (G) liquid condensed (1,C) phase transition, Above 6.5 degrees C, the phase transition at zero surface pressure is followed by another phase transition. which is indicated by the presence of cusp point-, in the pi-t curves at different temperatures. Each of the cusp points is followed by it Plateau. Much is accompanied by two Surface phases, indicating that the latter transitions are also first-order in nature. At > 6.5 degrees C, the former transition is classified as a first-order G-liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE-LC phase transitions The critical surface pressure (pi(c)) necessary for the G-LC and G-LE phase transitions is zero and remains constant all mer the studied temperatures, whereas that for the LE-LC transition increases linearly with increasing temperature. Based oil these result,,, we construct it rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 degrees C. All the results are consistent with the present understanding or the Langmuir monolayers of insoluble amphiphiles at the air-water interface. (c) 2005 Elsevier Inc. All rights reserved.