High-field electron paramagnetic resonance spectra were collected at several frequencies for a single crystal of [Zn3.91Ni0.09(hMP)(4)(dmb)(4)Cl-4] (1), where dmb is 3,3-dimethyl-1-butanol and hmp(-) is the monoanion of 2-hydroxy-methylpyridine. This crystal is isostructural to [Ni-4(hmP)(4)(dmb)(4)Cl-4] (2), which has been characterized to be a singlemolecule magnet (SMM) with fast quantum tunneling of its magnetization (QTM). The single Ni-II ion zero-field-splitting (zfs) parameters D-i [= -5.30(5) cm(-1) and E-i [= +/- 1.20(2) cm(-1)] in the doped complex 1 were evaluated by rotation of a crystal in three planes. The easy-axes of magnetization associated with the single-ion zfs interactions were also found to be tilted 150 away from the crystallographic c direction. This inclination provides a possible explanation for the fast QTM observed for complex 2. The single-ion zfs parameters are then related to the zfs parameters for the Ni-4 molecule by irreducible tensor methods to give D = -0.69 cm(-1) for the S = 4 ground state of the SMM where the axial zfs interaction is given by D (S) over cap (2)(z).