We compared the thermodynamic behavior of supported and free-standing films of phospholipids with different chain length and showed that the change in free energy for the phase transition to a denser state was greater in free-standing than in supported membranes, with the differences being independent of the chain length. The presence of the support promoted a decrease in the energy gap between the coexisting phases of 1.7 kJ mol(-1), and this was a long-range effect affecting both leaflets, probably related with the impediment of out-of-plane motion of the film by the solid surface.