The surface pressure-temperature diagram of Langmuir monolayers of mixtures of heneicosanoic acid with heneicosanol has been determined from 10 degrees C to 40 degrees C by direct observation with Brewster angle microscopy. The measurements focused on the way in which the boundary between the L(2)’ and L(2) phases change with composition. As previously observed by Shih et al. [J. Chem. Phys. 101, 9132 (1994)], it moves to lower pressure and higher temperature with increasing concentration of alcohol. We have discovered that the boundary between the Ov and L(2) phases, which had not been studied before, moves in the opposite direction and that the L(2)’ and Ov phase regions merge, an unanticipated result.