The behavior of a reverse lamellar phase has been studied by small-angle X-ray scattering upon the insertion of triblock molecules. A decrease of the lamellar spacing and membrane thickness was observed, whereas the Caille exponent remained constant whatever the concentration of triblock molecules. A similar behavior was observed when surfactant molecules were added to the lamellar phase instead of triblock molecules. We demonstrate that our experimental results are consistent with the assumption that triblock molecules only participate to increase the membrane surface area. Moreover, we emphasize the fact that the surfactant molecular area varies with the membrane thickness. Indeed, this property appears to be the main effect causing the observed dramatic decrease of the lamellar spacing.