Interaction between an air-stable orange-colored dye molecule, acridine orange base (AOB), and single-crystal surfaces of ZnO(101-0), (0001), and (000-1) is examined by synchrotron-radiation-excited photoelectron spectroscopy and X-ray absorption spectroscopy. AOB adsorbs molecularly on the ZnO surfaces to form chemical bonds with the surface O atoms. AOB tends to lie flat on the surfaces, whereas a more stand-up orientation is preferred on the AOB-crowded surfaces. AOB adsorption induces downward bending of the ZnO band with the largest amount on ZnO(000-1), followed by the ZnO(10-10) and ZnO(0001) surfaces, indicating an electron donor property of AOB. The energy level alignment at the AOB/ZnO interface is determined and is found to be characterized by a type-II heterojunction, which favors the charge transfer of the excited electron from AOB to ZnO.