Spin-resolved photoemission and absorption studies of Fe ₃ O ₄ and CrO ₂ epitaxial thin films have been reviewed to address the relationship between the electron correlation effects and the half-metallic properties of these two materials. Spin-resolved photoemission results suggest that Fe ₃ O ₄ should be considered as a strongly correlated system, and that Fe ₃ O ₄ is not a half-metal. Spin-resolved O 1s X-ray absorption measurements on ferromagnetic CrO ₂ reveal that the spin polarization of the unoccupied states closest to the Fermi level approaches 100%, confirming the half-metallic ferromagnetic nature of the material. The spin polarization of the main line of the unoccupied states, on the other hand, is found to be only 50%, indicating a very atomic-like behavior of the Cr 3d² ions.