Comparative study of photosynthetic reaction centres (RC) and bacteriorhodopsin (BR) in solution and LB films leads to the conclusion that the molecular close packing plays a key role in the thermal stability of proteins. LB organization of both RC and BR results in the strong improvement of the thermal stability, while when the films are prepared by only drying the solutions, the behaviour of RC and BR is different. RC reveal a decreased thermal stability, while in the case of BR there is no difference with respect to LB film. The last fact can be explained by taking into consideration the fact that BR molecules are organized into closely packed fragments of purple membranes in solution.