The thermal stability and structure of two uniform Langmuir - Blodgett films made of bipolar lipids from archaea have been studied. X-ray diffraction measurements at various temperatures from 20°C to 100°C were performed on these films, during heating and cooling cycles. The Bragg peak position, the integrated intensity and the peak width have been recorded as a function of temperature. Both intensity of diffraction peaks and layer thickness diminish at high temperatures while the correlation length, normal to the multilayer plane, displays some significant changes during the heating cycle. The data suggest the occurrence of some phase transitions. As a final result, the thermal stability of these two bipolar lipids LB-films is quite different and very high for the Phospho 2 LB film, which is stable till 100°C and is very promising for technological applications.