Ni(II)-substituted rubredoxins from Desulfovibrio gigas, Desulfovibrio vulgaris, and Clostridium pasteurianum have been prepared and characterized by room-temperature UV-visible absorption and low-temperature MCD spectroscopy. The results are compared with those obtained for the crystallographically defined analogue complex (Ph4P)2Ni(SPh)4. The Ni(II) sites in both the biological and inorganic compounds are found to be high spin, with very similar electronic and magnetic properties. The results are interpreted in terms of tetragonally distorted tetrahedral thiolate coordination for Ni(II), resulting in a 3A2 ground state, under D2d symmetry, that is subject to large axial zero-field splitting. Studies of the temperature dependence of the solution MCD spectra facilitate quantitative assessment of the axial zero-field splitting parameters; D = 55 cm-1 for Ni(II)-substituted Rd and D = 44 cm-1 for Ni(SPh)42-. The properties of Ni(II) sites in hydrogenases are discussed in light of these results.