We have studied the tilt angle of Langmuir monolayers of octadecanol (CH3(CH2)(17)OH) using Brewster angle autocorrelation spectroscopy. Measurements were carried out at the triple point of the next nearest neighbor tilted L-2’, the untilted distorted hexagonal LS(Rot I), and the crystalline herringbone S phase and at the triple point of the next nearest neighbor tilted L-2’, the untilted distorted hexagonal LS(Rot I), and the hexagonal undistorted LS(Rot II) phase, We show that the transition from the tilted phase to the untilted phases as a function of temperature changes from second order (L-2’S) to first order (L-2’/LS(RotI)) to second order (L-2’/ LS(Rot II)) and that the S/LS(Rot I) and LS(Rot I)/LS(Rot II) phase transition lines at high pressure continue into the tilted L-2’ phase. The L-2’ phase actually consists of three different phases of different tilt angle with some similarity to the phase behavior of fatty acids. Strong superheating and supercooling effects prevent a direct observation of both phase transitions.