We address the issue of the origin of the bending rigidity of a charged membrane formed from amphiphilic molecules. Electrostatic effects are investigated by direct measurement of the force necessary to deform a catanionic membrane as function of the ionic strength of the medium by means of an atomic force microscope (AFM). Using continuum mechanical modeling of membrane deformation, we derive the bending rigidity of the catanionic membranes and monitor for the first time its decrease in response to increasing salt concentration.