Magnetic properties of bis(acetato-O)tetrakis(imidazole-N3)nickel(II) have been investigated by low-temperature magnetic susceptibility as well as high-field magnetisation and subjected to theoretical analysis. A simultaneous fitting of susceptibility and magnetisation gave the following set of magnetic parameters: g(z) = 2.181, g(x) = 2.039, and D/hc = -22.3 cm(-1). Thanks to the magnetisation measurements the sign of the axial zero-field splitting parameter is fixed unambiguously. The negative value of the D-parameter, unexpected for the geometry of an elongated tetragonal bipyramid, is understandable by introducing different orbital reduction factors for axial (acetate) and equatorial (imidazole) ligands. (C) 2003 Elsevier Science B.V. All rights reserved.