Results are presented from an experimental study of stall flutter oscillations from a two-degree-of-freedom, pitch/plunge airfoil system with nonlinear structural stiffness in the plunge direction. With linear (only) structural stiffness, the airfoil system could exhibit a large-pitch-amplitude limit cycle response which is attributed to stall. With the addition of the nonlinear stiffness, the airfoil system could exhibit two classes of limit cycle response: one with low-pitch-amplitude attributed to the structural nonlinearity and one with high-pitch-amplitude attributed to stall. The amplitudes of the limit cycles for cases in which the structurals and aerodynamic nonlinearities co-exist are modulated and remain steady over a range of airspeeds and for initial excitations above a critical value.