Magnetic properties of dinudear nickel(II) complex [Ni-2(sym-hmp)(2)] (BPh4)(2).3.5DMF.0.5 (2-PrOH) (1), where (symhrnp)(-) is 2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methyl-phenolate anion and DMF indicates dimethylformamide, were investigated using high-frequency and-field electron paramagnetic resonance (HFEPR). To magnetically characterize the mononuclear nickel(II) species forming the dimer, its two dinudear zinc(II) analogues, [Zn-2(sym-hmp)(2)](BPh4)(2).3.5DMF.0.5(2-PrOH) (2) and [Zn-2(sym-hmp)(2)](BPh4)(2).2acetone.2H(2)O (2'), were prepared. One of them (2') was structurally characterized by X-ray diffractometry and doped with 5% mol nickel(II) ions to prepare a mixed crystal 3. From the HFEPR results on complex 1 obtained at 40 K, the spin Hamiltonian parameters of the first excited spin state (S = 1) of the dimer were accurately determined as vertical bar D-1 vertical bar = 9.99(2) cm(-1), vertical bar E-1 vertical bar = 1.62(1) cm(-1), and g(1) = [2.25(1), 2.19(2), 2.27(2)], and for the second excited spin state (S = 2) at 150 K estimated as vertical bar D-2 vertical bar approximate to 3.5 cm(-1). From these numbers, the single-ion zero-field splitting (ZFS) parameter of the Ni(II) ions forming the dimer was estimated as vertical bar D-Ni vertical bar approximate to 10-10.5 cm(-1). The HFEPR spectra of 3 yielded directly the single-ion parameters for D-Ni = +10.1 cm(-1), vertical bar EN1 vertical bar = 3.1 cm(-1), and g(iso) = 2.2. On the basis of the HFEPR results, the previously obtained magnetic data (Sakiyama, H.; Tone, K.; Yamasaki, M.; Mikuriya, M. Inorg. Chim. Acta 2011, 36S, 183) were reanalyzed, and the isotropic interaction parameter between the Ni(II) ions was determined as J = -70 cm(-1) (H-ex = -JS(A).S-B). Finally, density functional theory calculations yielded the J value of -90 cm(-1) in a qualitative agreement with the experiment.