We have investigated the interlayer exchange coupling between the epitaxial spinel Fe3O4 layers across an insulating nonmagnetic MgO spacer. The epitaxial structure used for these investigations was Fe3O4 (10 nm)/MgO (0.8–3 nm)/Fe3O4 (10 nm)/NiO (15 nm) multilayers grown on MgO (100) substrates. We find that the two Fe3O4 layers are antiferromagnetic coupled through the MgO spacer when the MgO thickness is less than 1.5 nm. Furthermore, ab initio calculation of IEC for Fe/MgO/Fe indicates the importance of electrode states, in particular, partial oxidation of the ferromagnetic electrodes. ; PUBLISHED ; This work was supported by the SFI under Contract No. 06/IN.1/I91. O.N.M. is grateful to the SFI C.T. Walton fellowship support during his stay at TCD. We are grateful to Dr. Heinonen for useful discussion.