X-ray diffraction is used to determine the structure of Langmuir-Blodgett (LB) films of copper phthalocyanine derivatives. It is shown that molecules aggregate in columns lying flat on the substrate surface. The phthalocyanine subunits are stacked in an eclipsed manner within each column with a periodicity of 3.38 Angstrom; an intracolumnar correlation length of ca. 90 Angstrom is found. A perfect interlayer registry is found, reflected by the formation of a square columnar phase. A similar organization is found in the solid phase. The lattice parameters as well as the texture of the LB films are precisely determined. The structural effects of a mild annealing on these structural data is studied.