A series of water-bridged dinickel complexes of the general formula [Ni2(?2-OH2)(?2-O2CtBu)2(O2CtBu)2(L)(L?)] (L = HO2CtBu, L? = HO2CtBu (1), pyridine (2), 3-methylpyridine (4); L = L? = pyridine (3), 3-methylpyridine (5)) has been synthesized and structurally characterized by X-ray crystallography. The magnetic properties have been probed by magnetometry and EPR spectroscopy, and detailed measurements show that the axial zero-field splitting, D, of the nickel(II) ions is on the same order as the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange interaction can be related to the angle between the nickel centers and the bridging water molecule, while the magnitude of D can be related to the coordination sphere at the nickel sites.