Phase transition phenomenon of the 1-dodecanol monolayer at the air/water interface was studied by the dynamic gamma(t) curves and the adsorption isotherm obtained by ellipsometry at 20 degrees C. The surface-concentration adsorption isotherm clearly showed three abrupt increases at bulk concentration C of 1.3 x 10(-9), 2 x 10(-9) and 3.7 x 10(-9) mol/mL, respectively. The 1st and the 3rd transitions observed herein, that were typical 2D first-order transitions, were consistent with the gas to liquid expanded (G-LE) and the liquid expanded to liquid condensed (LE-LC) phase transitions observed in a previous tensiometry study. The 2nd transition that occurred at C = 2 x 10(-9) mol/mL was not identified from any previous dynamic surface-tension profiles. Judging from the substantial increase in the film thickness of the transition, it was believed that the orientation change of the adsorbed molecule was involved in the LE phase. A LEh and a LEv phase, that denoted the "lie-down" and "stand-up" types of adsorption, respectively, was used to describe this transition and a cusp, instead of a constant surface-tension region, was observed in the dynamic gamma(t) curves for this transition. This suggested that, since the surface tension varied during the transition process, the newly identified LEh and LE, transition might not be the typical first-order type of phase transition. (C) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.