Brewster angle autocorrelation spectroscopy (BAAS), a combination of Brewster angle microscopy and the photocorrelation technique is a new method for the quantitative characterization of Langmuir monolayers. In this paper we compare tilt angle measurements in octadecanol monolayers performed using BAAS and grazing incidence x-ray diffraction (GIXD). BAAS offers the possibility of tilt angle measurements in Langmuir monolayers—without any fitted parameters—with a precision similar to GIXD, but ten times faster. GIXD, conversely, offers detailed microscopic information not available from BAAS. The swiftness of BAAS is exploited for a detailed analysis of the phase transitions from the L2′ phase to the LS (Rotator I) and LS (Rotator II) phases. The L2′/LS (Rot I) transition is of first order whereas the L2′/LS (Rot II) transition is more probably of weak first order than of second order.