Humidity-induced oxidation has been considered as a major cause for the growth of dark spots in organic electroluminescent devices. In this Letter, we found that a layer of 100 nn thick calcium deposited on top of the magnesium:silver (Mg:Ag) cathode can slow down the growth rate of dark spots. Further experiments verified that calcium not in direct contact with the cathode could not protect the dark spots against growing. This excludes the possibility that the protective function of calcium derives solely from its action as a moisture-resistive agent. In contrast, calcium functions as a sacrificial metal forming a galvanic couple with the magnesium electrode such that calcium would oxidize in preference to the Mg:Ag.