Reported here are the structural properties of a zinc 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine Langmuir monolayer on a water surface under progressive lateral compression investigated by grazing incidence X-ray scattering. Grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) is exploited to determine specular reflectivity-like information where the phase change of flat-lying molecules on the surface to edge-standing molecules perpendicular to the surface is directly observed. Furthermore, grazing incident X-ray diffraction (GIXD) is used to investigate the in-plane ordering of the system where it has been found that in the high-density state (approximately 0.35 nm(2) per molecule) the system can be considered to be a monolayer consisting of arrays of side-by-side lying cofacially aggregated cylindrical rodlike entities.