We used single crystals of [A(+)(C5H5)(4)](2)C60-B- as an ideal model system to determine accurately principal values and principal axes of the g-tensor of the C-50 mono radical anion embeded in a crystal field. The experimentally observed non axial g-tensor which corresponds to the molecular Jahn-Teller distortion of C-60(-) shows that the crystal field has stabilized the Jahn-Teller distortion of D-2h symmetry along a specific molecular C-2 axis. These stabilization occurs below a characteristic temperature T-s for two C-60(-) orientations (merohedral disorder) and depends on the cations as we could show by the substitution of pf for As+.