The dinuclear iron cluster of the oxidized hydroxylase component of methane monooxygenase (MMOH) contains two antiferromagnetically coupled high-spin ferric ions (H = JS(A) . S-B, S-A = S-B = 5/2, J = 15 cm(-1)). Previous Mossbauer studies revealed that the electronic ground state of the cluster contains a paramagnetic admixture; this is manifested in magnetic hyperfine splittings that are larger by about 10% than those attributable to the applied magnetic field. This observation cannot be explained by anisotropic Zeeman interactions, zero-field splittings, or anisotropic exchange. Here we report Mossbauer and magnetization studies of the (mu-phenoxo)bis(mu-carboxylato)diiron(LI complex, [Fe-2(HXTA)(O2CCH3)(2)](-), 1; HXTA = N,N'-(2-hydroxy-5-methyl-1,3-xylylene)bis(N-carboxymethylglycine). Like MMOH, complex 1 contains a pair of antiferromagnetically coupled high-spin ferric ions tour magnetization data yield J = 20 +/- 2 cm(-1)), and its 4.2 K Mossbauer spectra also exhibit increased magnetic splittings. Studies of the (GaFeIII)-Fe-III analogue of 1 revealed no unusual properties of the high-spin Fe(III) site, suggesting that the increased magnetic splittings are attributes of the pair rather than properties of the local sites. The Mossbauer spectra of 1 recorded in parallel applied field exhibit nuclear Delta m = 0 transitions, indicating the presence of an interaction that produces at each iron site spin expectation values in directions perpendicular to the applied field. Analysis of the data of 1 shows that the unusual spectral features of the complex, and by extension those of MMOH, reflect the presence of antisymmetric exchange, d . (S-A x S-B); We Obtained \d\ = 2.2 +/- 0.7 cm(-1) for complex 1 and \d\ approximate to 1.5 cm(-1) for MMOH. This study shows that Mossbauer spectroscopy can be a sensitive tool for determining antisymmetric exchange interactions.