The effect of barrier Al mole fraction, 0.2 less than or equal to x less than or equal to 0.8, on tunneling currents has been studied for a set of asymmetric GaAs/AlxGa1-xAs double barrier structures. The barrier widths of each sample were scaled so that barrier transmission coefficients for different samples should be approximately equivalent at the first resonant tunneling peak. Structures were grown by molecular beam epitaxy; by adjusting Ga and Al cell temperatures, the full range of Al mole fractions could be achieved in AlxGa1-xAs barrier layers while maintaining a nearly constant growth rate of about 1 mu m/h. Current-voltage measurements are in agreement with theoretical estimates and indicate good sample quality.