The dodecametallic Cr(III) cluster [Cr12O9(OH)(3)(O2CCMe3)(15)] has a ground spin state of S = 6 characterized by the spin Hamiltonian parameters g(ZZ) = 1.965, g(XX) = g(YY) = 1.960, D-S=6 = +0.088 cm(-1), and E-S=6 = 0 (where D and E are the axial and rhombic zero-field splitting parameters, respectively) as determined by multifrequency EPR spectroscopy and magnetization studies. Micro-SQUID magnetization studies reveal steps due to the fine structure of the ground state, with the spacing between the steps in excellent agreement with the D-S=6 value determined by EPR. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion g values and D value (= -1.035 cm(-1)) of the constituent Cr(III) ions directly. A vector coupling analysis demonstrates that the cluster ZFS is almost entirely due to the single-ion component. Thus, the relative orientations of the local and cluster magnetic axes can lead to a cluster ZFS of opposite sign to the single-ion value, even when this is the only significant contribution.