Aqueous solutions of zirconium oxychloride aged at temperatures between 298 and 333 K for 0-26 days were investigated. Nanoparticles with different size distributions depending on aging temperature and duration were detected by dynamic light-scattering measurements. Extended X-ray absorption fine structure analysis and scanning transmission electron microscopy measurements revealed the nanoparticles formed in the samples to be zirconium polynuclear complexes and zirconium dioxide nanocrystals under mild condition, which was previously unexplored.