NiO supported on zirconia modified with MoO3 for ethylene dimerization was prepared by drying powdered Ni(OH)(2)-Zr(OH)(4) with ammonium heptamolybdate aqueous solution, followed by calcining in air at high temperature. The characterization of prepared catalysts was performed using FTIR, Raman, XRD, and DSC. MoO3 equal to or less than 15 wt% was dispersed on the surface of catalyst as two-dimensional polymolybdate or monomolybdate, while for MoO3 above 15 wt%, a crystalline orthorhombic phase of MoO3 was formed, showing that the critical dispersion capacity of MoO3 on the surface of catalyst is 0.18 g/g NiO-ZrO2 on the basis of XRD analysis. From 600 degrees C of calcination temperature, zirconium molybdenum oxide, Zr(MoO4)(2) was formed due to the reaction between ZrO2 and MoO3; its amount increased with the calcination temperature. The acid amount and the catalytic activity of catalysts increased with the amount of dispersed MoO3. The high acid strength and acid amount was responsible for the Mo=O bond nature of the complex formed by the interaction between MoO3 and ZrO2. The catalytic activity for ethylene dimerization was correlated with the acid amount of the catalysts measured by the ammonia chemisorption method. (c) 2006 Published by Elsevier B.V.