Two phase transitions, G-LE and LE-LC, of the adsorbed monolayer of the soluble surfactant 1-dodecanol at the air-water interface were observed from the dynamic surface tension measurement by using a pendant bubble tensiometer. Two constant surface-tension (gamma) regions were detected in the dynamic surface-tension gamma(t) profiles of different surfactant concentrations at 20 degreesC. They are at 7 = 0.75 and 17.9 mN/m. A constant gamma region in dynamic gamma(t) profiles indicates the existence of a 2D phase transition. These 2D phase transitions of adsorbed layer are presumed to be the coexistence of the gas (G) and liquid-expanded (LE) phases and that of the LE and liquid-condensed (LC) phases. A new phase-transition isotherm, which is possible to describe the 1-dodecanol adsorbed monotayer, is also proposed.