The precursor of zirconium oxide is synthesized by the sol-gel method by reacting zirconyl chloride octahydrate (ZrOCl2 . 8H(2)O) and citric acid in ammonia solution. The ionized complex compounds (R(COO-+NH4)(3)) were first formed from the reaction of complex agent (citric acid) and ammonium hydroxide. Zirconyl chloride octahydrate first hydrolyzed to produce zirconium tetranuclear complex molecule [Zr-4(OH)(8)(OH2)(16)(-OOC)(3)R](8+), which subsequently produced a large complex molecule ([Zr-4(OH)(5)(OH2)(16)(OOC)(3)R](5+) or [Zr-4(OH)(8) (OH2)(13)(-OOC)(3)R](5+)) through the polymerization of zirconium tetramer with the ionized complex agent. The effect of the reaction conditions on the precursor solution's viscosity and average particle size were investigated in order to understand the kinetics of the reaction. A rational reaction mechanism is proposed satisfactorily to explain the formation and growth of the particles and viscosity of the sol.