The magnetic anisotropy of the nona-Cu(II) [3 x 3] grid structure [Cu-9(2POAP-H-6)](NO3)(12). 9H(2)O was investigated by means of torque magnetometry. The analysis of the high-field part of torque vs. field curves measured at 1.75 K allowed a precise determination of all anisotropy parameters of the ground state. These could be related to the magnetic parameters of the microscopic Hamiltonian by a suitable perturbation theoretical treatment. We found strong evidence for a sizeable anisotropic exchange interaction. The observed anisotropy of the g-factor could not be explained by simple ligand field arguments which is attributed to the unusual quasi-octahedral coordination environment of the Cu ions.