Dielectric dispersion of a Haldane compound nickel trimethylenediamine nitrite perchlorate Ni(C 3 H 10 N 2 ) 2 NO 2 ClO 4 (NINO) has been measured around the structural phase transition at 378.5 K. The dielectric constant varies like a step at the phase transition and the dielectric dispersion of Debye type was observed along an axis perpendicular to the electrically polar axis above T c .