A very selective, simple, sensitive fluorimetric method for the determination of oxalate ion is described. The method is based on the quenching by oxalate of the fluorescence of a 1 : 1 zirconium-flavonol chelate in dilute sulfuric acid solution. The fluorescence is measured at 460 nm. A linear calibration curve is obtained for 0-10 μg of oxalate ion; there are very few interferences. © 1969.