It has been recently reported that alkyl esters of acyl-L-carnitine show a lyotropic liquid crystalline polymorphism. Depending on the relative length of the hydrocarbon chains, two different phase behaviours were detected : in addition to the lamellar L alpha phase, compounds bearing a relatively short alkyl chain exhibit hexagonal H and cubic Ia3d type I phases. In this paper, X-ray diffraction and osmotic stress experiments have been performed in order to directly measure as a function of distance the forces between structure elements, i.e., lamellae in the L alpha phase and rods in the H phase. In both cases, exponentially decaying repulsive forces dominate the interactions at close separation. The magnitude and decay length of these forces are very similar. The exponential nature of the repulsion and the value of the decay length led us to conclude that the dominant force experienced by the structure element surfaces approaching contact is hydration repulsion. This force represents the work of removing water from the vicinity of the approaching surfaces.