The compression isotherms of copper tetrakis (n-alkyloxycarbonyl) phthalocyanines present a well-defined monolayer to bilayer transition. The structure of Langmuir-Blodgett films built from both types of Langmuir film is investigated by X-ray diffraction using one representative compound of the series. A similar square columnar organization of the molecules is found in both cases. Conclusions concerning the structure of the bilayer at the air-water interface are drawn.