We assess the potential of the ferrimagnetic spinel ferrites CoFe 2 O 4 and NiFe 2 O 4 to act as spin filtering barriers in magnetic tunnel junctions. Our study is based on the electronic structure calculated by means of first-principles density functional theory within different approximations for the exchange correlation energy. We show that, in agreement with previous calculations, the density of states suggests a lower tunneling barrier for minority spin electrons, and thus a negative spin-filter effect. However, a more detailed analysis based on the complex band structure reveals that both signs for the spin-filtering efficiency are possible, depending on the band alignment between the electrode and the barrier materials and depending on the specific wave-function symmetry of the relevant bands within the electrode ; PUBLISHED ; This work has been supported by Science Foundation Ireland under (Grants SFI-07/YI2/I1051 and 07/IN.1/I945) and by the EU-FP7 (iFOX project). We made use of com- putational facilities provided by the Trinity Centre for High Performance Computing (TCHPC) and the Irish Centre for High-End Computing (ICHEC)