Re- and PGE-bearing magnesioferrite spinels were synthesized at oxidizing conditions between 0.1 MPa and 5.0 GPa, and 1150 to 1600 ¿C in equilibrium with Re- or PGE-bearing metals and/or oxides. Electron microprobe analysis and single crystal X-ray diffraction techniques were employed to determine the composition, crystal structures and site occupancies, indicating that the magnesioferrite structure can accommodate wt% levels of Re and the platinum group elements (PGE) at oxidized conditions. These results suggest that magnesioferrite spinels found in K-T boundary samples worldwide could be an important host phase for the Ir, Ru, Os, Re, and Rh anomalies found in the boundary layer. Higher Ru/Ir values in Pacific magnesioferrite-bearing samples may be a reflection of higher condensation temperatures of the oxides. The distribution of PGE's in mantle and magmatic rocks may depend on the stability of spinel-structure oxides, especially those with a high magnesioferrite component. Finally, magnetic-rich meteorite fusion crusts may hold PGEs in oxidized form. ¿ 2001 American Geophysical Union