The formation rate and primary particle size of monoclinic, hydrous zirconia particles produced by the hydrolysis of various ZrOCl2 solutions (with and without the addition of HCl, NH4OH, NaCl, CaCl2, or AlCl3) were measured to clarify the effects of the H+ and Cl- ion concentrations on the nucleation and crystal-growth processes of primary particles of hydrous zirconia. Chemical-kinetic analyses, to which Avrami-Erofeev's equation was applied, and XRD measurements revealed that both the rate constant and the primary particle size of the hydrous zirconia decreased as the concentrations of H+ and/or Cl-ions produced by hydrolysis increased. The nucleation rate per unit of ZrOCl2 concentration and the crystal-growth rate of the primary particles of the hydrous zirconia were determined by analyzing the relationships between the rate constant and primary particle size. The nucleation rate per unit of ZrOCl2 concentration revealed almost no change and remained constant as the H+ and/or Cl- ion concentrations increased, except in the case of a slight increase for ZrOCl2 solutions with added HCL The crystal-growth rate decreased as the H+ and/or Cl-ion concentration increased. The present kinetic analyses revealed that the decrease in rate constant with increasing H+ and/or Cl- ion concentrations resulted from the decrease in the crystal-growth rate. The decreasing tendency of the crystal-growth rate was attributed to interference with crystal growth by the Cl- ions attracted onto the particle surface through the formation of an electric double layer. The formation mechanisms for the primary particles of hydrous zirconia were determined based on the present experimental results.