Ballistic electron emission microscopy (BEEM) has been used to study electron transport across single barrier AlxGa1-xAs/GaAs heterostructures. The structures, grown by molecular beam epitaxy, utilized a p-type delta-doped sheet to cancel the band bending near the Schottky interface, enabling a direct measurement of the conduction band offset at room temperature. The band offset at room temperature for x=0.21 is 0.19 eV and for x=0.42 is 0.33 eV. Measurements at 77 K gave values of 0.20 eV for x=0.21 and 0.35 eV for x=0.42. These results demonstrate that BEEM can be used to probe the transport properties of semiconductor heterostructures which are spatially beneath the Schottky barrier.