Thin films of metallo-porphyrins have interesting properties which appear to be related to the film structure. Mixed Langmuir-Blodgett (LB) films of tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(TDBPC) with cadmium arachidate have been studied by grazing incidence X-ray diffraction (GIXD) to investigate their structures and miscibilities. In the two-component films there are two 2-dimensional structures present in the film plane, with geometries and correlation lengths that are independent of the molar ratio of the two compounds. One is from cadmium arachidate domains, and has a rectangular symmetry (alpha = 7.44 Angstrom b = 4.95 Angstrom, and gamma = 90 degrees). The other is from the porphyrin domains and has an oblique symmetry (alpha = 15.2 Angstrom, b = 8.61 Angstrom, and gamma= 80.3 degrees). The molecular chains of cadmium arachidate and the macrocycles of the porphyrin are perpendicular to the film plane. The area per porphyrin molecule is 124 Angstrom(2), consistent with the value obtained from surface pressure-area measurements on the floating monolayers. The components are immiscible. There is no structural correlation between the layers in the LB films, and only those films with a high cadmium arachidate content exhibit a clear lamellar structure. The presence of long-chain alkane molecules (so-called trigger molecules) in the films did not alter the molecular orientation in the porphyrin domains, but did disturb the domain orientation with respect to the surface normal of both materials.