Adsorption of pyridine on Nin-clusters (with n = 2,3,4) is studied by quantum chemical calculations at B3LYP/LANL2DZ and B3LYP/6-311G** levels. First, Nin-clusters are investigated for accessible structure and electronic states. The lowest electronic state with four unpaired electrons is predicted for Ni4-cluster based on geometry and electronic structure, showing that the cluster stability nicely depends on number of unpaired electrons. Correction for basis set superposition error of metal-metal bond is appreciable and has increasing effect on cluster binding energy. Next, adsorption of pyridine in planar and vertical adsorption modes is investigated on rhombus Ni4-cluster. The vertical mode is found (at B3LYP/6-311G** level) as the most favorable adsorption mode. Adsorption energy (ΔEads) depends on cluster size; adsorption on Ni4-cluster is most favorable with ΔEads = −207.33 kJ/mol. The natural bond orbital analysis reveals the charge transfer in adsorbate/metal-cluster. Results of investigations for the Ni2- and Ni3-cluster are also presented. © 2012 Wiley Periodicals, Inc.