The interfaces between metal and arylamine molecular films were studied by measurement of current-voltage characteristics in single-layer organic diodes with the aid of the anode interfacial engineering method. The diode turn-on voltage is shown to be very sensitive to the arylamine/metal interface and thus can serve as a means for characterizing such organic/metal interfaces. We show that the diode electrical performance depends on the arylamine substituent groups and chemical/electrical properties of the anode. The conduction mechanism of the diodes is shown to be injection limited, which can be well described with the Richardson-Schottky thermionic emission model. This work may provide some insight into the use of single-layer organic diodes and the interfacial engineering method to rapidly probe the organic/metal and even organic/organic interfaces.