In this study, we have characterized graphene oxide films formed at the air–water interface by X-ray reflectivity and grazing incidence X-ray diffraction using synchrotron sources. Surprisingly, the results of both measurements show that at non zero surface pressures, the film is organized as a bilayer of sheets interfaced between air and water with water molecule bridges. Such a spontaneous bilayer structure and its evolution with respect to the surface pressure has been observed for the first time. These results should allow precise control of the density of sheets deposited on the substrate when these films are transferred through the Langmuir Blodgett or Schaefer procedures. Indeed, graphene oxide keeps on attracting more and more attention, increasing the need for the production of well-controlled graphene oxide thin films due to its application in energy devices or in sensor domains.