Corresponding to their relative low work function, Ca and Mg are interesting metals for cathodes in organic light emitting devices. In this study, the interaction of these metals with the blue phosphorescent emitter material Ir(cnpmbic)3 is investigated by in-situ infrared spectroscopy. Thin films of the organic material are deposited by vapour sublimation under ultra-high vacuum conditions. Further deposition of Ca on the organic layer at room temperature gives rise to new features in the infrared spectrum of the sample. The new features indicate diffusion of Ca into the organic layer and do not appear at much lower temperature (110 K). They are attributed to dynamic charge transfer processes that may occur at rough metal surfaces. On the other hand, Mg deposited at room temperature does not stick on the organic material.