We have performed grazing incidence X-ray diffraction studies of Langmuir monolayers of an azobenzene derivative. There are only two phases over a range of temperatures and pressures in which at least seven phases exist in monolayers of simpler molecules (0-30 degrees C, 0-40 dyn/cm). The low-pressure phase has tilt toward a nearest neighbor, while the high-pressure phase has tilt toward a next-nearest neighbor. Within each phase the tilt angle does not change significantly wit h pressure. This behavior is very different from that of monolayers with saturated hydrocarbon tails and implies that the azobenzene groups ＇lock＇ and prevent the molecules from sliding against each other. However, the lattice parameters change with pressure; this suggests that the molecular conformation changes. The structure of the low-pressure phase is similar to one observed in ate-derivative bilayer membranes and crystals, while the high pressure phase has a structure not seen before in such systems.