In this paper we present an improved method for the selective and sensitive determination of oxalate in different matrices such as urine, plasma, and food. The method uses ion chromatography for the separation of anions. To overcome problems with interfering matrix-anions, colourings, and macromolecules, we used an inline enzyme-reactor (ER) containing immobilised oxalate oxidase, which converts oxalate to hydrogen peroxide. Hydrogen peroxide was analysed with high sensitivity by amperometric detection. The determination limit for the HPLC-ER method was 1.5 mumol/1, the mean recovery in urine was 102%. The evaluation in a urinary matrix achieved C.V. values from 2.2% to 6.7% for the within-run precision and C.V. values from 3.7% to 8.6% for the between-batch precision. The results of the new method were statistically equivalent to those obtained by enzymatic kits. We present first results of the HPLC-ER method, when applied to body fluids and food analysis.