Summary o-Phenylenedioxydiacetic acid has been found to be a selective reagent for the estimation of zirconium. As little as 2.1 mg of zirconium can be easily estimated. The composition of the precipitate varies somewhat and therefore, direct weighing is not possible. This difficulty is overcome by igniting as oxide. Be²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Hg²⁺, Al³⁺, Ce³⁺, Ti⁴⁺, Th⁴⁺, UO²⁺, Mn²⁺, Fe³⁺, Co²⁺, and Ni²⁺, ions do not interfere. Although V₂O₂⁴⁺ and Cr³⁺ ions are not precipitated in neutral or slightly acidic solutions they contaminate the zirconium precipitate, at about 0.30 N HCl concentration. The amount of contamination is so small that it is removed by double precipitation. This method gives satisfactory results even in the presence of small amounts of SO₄^2–ions.