We synthesized a mto (= monothiooxalato)-bridged heterometal complex, Ph4P[(ZnFeIII)-Fe-II(mto)(3))] consisting of (FeO3S3)-O-III and (ZnO6)-O-II octahedra and investigated its physical properties. Ph4P[(ZnFeIII)-Fe-II(mto)(3)] has a two-dimensional honeycomb network structure. The ESR and Fe-57 Mossbauer spectra revealed that the rapid spin equilibrium in which the high-spin (HS) state and the low-spin (LS) state exchange in the time scale of 10(-10) < tau < 10(-7)s occurs at the (FeO3S3)-O-III site, which is the first report for an assembled metal complex system.