According to experimental data on X-ray scattering and reflectometry with the use of synchrotron radiation, a two-dimensional crystallization phase transition in a monolayer of melissic acid at the n-hexane -water interface with a decrease in the temperature occurs after a wetting transition.The observation of a two-dimensional solid -liquid phase transition at the oil -water interface was reported in [1,2,3]. In this work, the temperature dependence of the intensity of diffuse (nonspecular) scattering of 15-keV photons at the n-hexane -water interface, where such a transition occurs in the adsorbed layer of melissic acid (C 30 -acid) [3], is studied. It is shown below that the diffuse scattering intensity in the low-temperature crystal phase at the interface is one or two orders of magnitude higher than that in the high-temperature phase, which indicates the existence of an extended transverse structure with a thickness of ∼ 200Å in the former phase. The analysis of experimental data within the theory of capillary waves indicates that the two-dimensional crystallization transition at the interface with a decrease in the temperature occurs after the wetting transition.Samples of macroscopically flat n-hexane -water interface were prepared and studied by the method presented in [4,5] in a stainless steel cell with dimensions of the interface of 75 mm× 150 mm * tikhonov@kapitza.ras.ru 1