This work describes a simple and low cost methodology for oxalate determination in food samples, which employs a bromothymol blue-based pH optode for the determination of CO2 generated in the enzymatic reaction between oxalic acid and oxalate oxidase. The enzyme was immobilised on barley seeds, together with catalase enzyme, and placed in a stirring bar type enzymatic reactor. The system showed a linear response range from 0.0080 up to 0.100 moll-1, when the measurements were carried out in 0.050 moll-1 succinate buffer at pH 4.0 and 25°C. In these conditions, the lifetime of the system was about 120 h, with a relative standard deviation <2% (four measurements of a 0.020 moll-1 oxalate solution). A value of 0.075 moll-1 was obtained for the apparent Michaelis-Menten constant, with a maximum velocity of 1908 μmolmin-1 for oxalic acid oxidation. No significant differences were found at a confidence level of 95%, when the results were compared with those obtained with the AOAC official standard method (974.24) for oxalate determination in spinach. © 2001 Elsevier Science B.V. All rights reserved.