The structures of mixed monolayers of cadmium arachidate (CdAr) with tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II) (TDBPC) (0.13 mole fraction) at the air/water interface have been studied by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity. Only the hexagonal structure of CdAr was observed in the GIXD measurements, with d(11) = 4.16 Angstrom, giving an area per chain of 20.0 Angstrom(2). The chains are perpendicular to the subphase surface and the positional correlation length is 45 Angstrom. For comparison, GIXD measurements of CdAr alone show a hexagonal in-plane structure with d(11) = 4.13 Angstrom and an area per chain of 19.7 Angstrom(2). The acyl chains in the film are perpendicular to the surface of the subphase and have a correlation length of 140 Angstrom. Reflectivity measurements on the monolayer of arachidic acid on a CdCl2 solution confirm that the chains are normal to the surface of the subphase and suggest that the headgroups have the composition -COOCdOH. For the mixed film reflectivity measurements indicate that, at a surface pressure of 25 mN m(-1), the porphyrin rings have a diameter of about 15.2 Angstrom and are perpendicular to the surface. At 2 mN m(-1) the rings are either tilted from the normal by about 28 degrees, or normal to the surface leaving some areas of low density in the film. The two compounds are immiscible in the mixed monolayer, but the formation of CdAr domains is hindered by the presence of the porphyrin.