The surfactant film bending elasticity can be described by a spontaneous curvature Co and two elastic constants K and ($) over bar K, associated with the mean curvature and the Gaussian curvature, respectively. These parameters are very important in the determination of the structure of the dispersions stabilized by the surfactant(droplets or sponge-like structures). We have studied ternary mixtures of oil, water, and nonionic surfactants of different chain lengths. Depending on the temperature, the microemulsions are in equilibrium with excess oil (oil in water structure), excess water (water in oil), or both excess oil and water (sponge-like structure). The interfacial tensions between the microemulsions and the excess phases are ultralow. We present neutron scattering experiments for droplet microemulsions, from which we determine the droplet radius and polydispersity. (Experiments were performed in Laboratoire Leon Brillouin, CE Saclay, 91191 Gif/Yvette, France, Laboratoire commun CEA-CNRS.) These results, in combination with interfacial tension determinations, are used to estimate K and ($) over bar K.