X-ray and ultraviolet photoemission spectroscopy investigations reveal strong interactions between Ca and tris-(8-hydroxy quinoline) aluminum(Alq3)during theCa/Alq3interface formation. The details of the interaction depend on the direction of the interface formation. For the case of Ca deposited onAlq3,a staged interface reaction is observed. For low Ca coverages(&THgr;Ca⩽4 Å),negatively chargedAlq3radical anions are formed by electron transfer from the Ca. The emergence of new states in the energy gap is observed in the UPS spectra. At higher coverages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of theAlq3molecule. On the other hand, for the case ofAlq3deposited on Ca, a strong chemical reaction takes place as soon asAlq3is deposited, and Ca attacks every constituent ofAlq3.Finally, no interaction occurs betweenAlq3and the Ca substrate if the substrate has been passivated by oxygen prior to theAlq3deposition. ©1998 American Institute of Physics.