The electronic structure of Fe(II) substituting Zn in Horse Liver Alcohol Dehydrogenase was investigated by Mössbauer spectroscopy at various temperatures and applied magnetic fields and by spin Hamiltonian analysis of the results. The novelty we found, is an unusually weak spin coupling of Fe(II) with a diradical (S=1). From ESR results and biochemical findings we conclude, that the corresponding chemical species is triplet oxygen (O2). Oxidation experiments, followed by Mössbauer spectroscopy, show that the spin-coupled species is an outer-sphere Fe(II)... O2 complex occuring as an intermediate of the dioxygen activation reaction, catalysed by Fe(II). A second Fe(II)-O2 complex could be detected, which corresponds to an inner-sphere complex with O2 directly bound to iron. The spin hamiltonian parameters in the coupled system describing the electronic properties of iron are presented. The results are compared with those of iron in other nonheme iron proteins.