An expression for the van der Waals free energy of an oil-water multilayer per one pair of oil-water lamellae is derived in the hypothesis of pairwise additivity. The formula has the expected symmetry in the thicknesses of both kinds of layers. This expression takes into account the interactions between the layers of the same kind and those of a different kind, as well as the intrinsic free energies of the layers. Such an expression can be useful in the thermodynamic treatment of lamellar liquid crystals.