Abstract The spontaneous geometrical characteristics and elasticity moduli of membranes, lipid monolayers, and vesicles are defined. The considerations are based on the general thermodynamical theory of mechanical properties of interfaces. The spontaneous area, spontaneous mean curvature, and spontaneous gaussian curvature are attributed to the state of a membrane with zero force factors. The elasticity moduli are defined as the derivatives of force factors. The general mechanical model of a membrane if suggested which takes into account the dependence of force factors on geometrical characteristics. The elastic properties of membranes composed of joined and freely sliding monolayers are compared. The spontaneous state of a spherical vesicle is analyzed.