M02ssbauer, EPR, and visible-region diffuse-reflectance techniques were used to characterize divalent and trivalent tris-(2,2′-bipyridine)iron complexes which had been synthesized within the large cavities of Y-type zeolites at low and high loadings. Exposure of samples containing the divalent Fe complex to chlorine gas produced the supported trivalent complex. The spectroscopic data for the low-loading iron complexes in the zeolite were similar to data obtained for Fe(bpy)3 complexes in other matrices; i.e., for [FeII(bpy)3]-Y(5.3) λmax = 528, 495, and 353 nm, IS = 0.626, and QS = 0.339 mm/s and for [FeIII(bpy)3]-Y(5.3) λmax = 612 nm, g⊥ = -2.60 ± 0.02, and g∥ = 1.66 ± 0.01. Differences were attributed to the probable higher negative charge density within the zeolite. The bipyridine ligand complexed 88% of the iron specifically in the tris form, and chlorine gas oxidized 89% of this divalent complex. With the high-loading sample, only 55% of the total iron was complexed by bipyridine since the iron content exceeded that necessary for one large tris complex per supercage. Excess Fe(II) was apparently close enough that it exerted an electron-withdrawing influence on the bpy ligands. Only 33% of the bpy complexes were effectively oxidized by chlorine gas, presumably due to exclusion of Cl2 or Cl- from certain cation sites.