This study focused on the investigation of the thermal stability of Fe on ZnSe and ZnS matrix using secondary ion mass spectroscopy (SIMS). Sandwiched three layer source structures of ZnSe/Fe/ZnSe and ZnS/Fe/ZnS were grown on GaAs and GaP substrates, respectively, by the molecular beam epitaxy technique. The bottom II-VI layers and the Fe-sandwiched layers in these structures are single crystalline while the top II-VI layers are polycrystalline. Thermal annealing was conducted in a range covering from 320 to 550 ° C. The SIMS depth profiles of the as-grown and annealed structures reveal that (100) oriented single crystalline Fe/ZnSe interface is thermally stable at temperature as high as 450 ° C while its polycrystalline counterpart suffers from fast diffusion even at the growth temperature. In contrast, (100) oriented polycrystalline Fe/ZnSe interface is quite stable at least up to 200 ° C. For both ZnSe/Fe/ZnSe and ZnS/ Fe/ZnS systems, (111) oriented structures were found to have lower thermal stability than (100) oriented ones. These results provide important findings towards the optimization of Fe-based tunneling magneto-resistance structures using a II-VI semiconductor barrier. © 2005 Published by Elsevier B.V.