An electrochemical approach to directly measure thedynamic process of H2O2 release from cultures of Arabidopsis thaliana cells is reported. This approach is based on H2O2 oxidation on a Pt electrode in conjunction with continuous measurement of sample pH. For [H2O2] <1mM, calibration plots were linear and the amperometric response of the electrode was maximum at pH 6. At higher concentrations ([H2O2] >1mM), the amperometric response can be described by Michaelian-type kinetics and a mathematical expression relating current intensity and pH was obtained to quantitatively determine H2O2 concentration. At pH 5.5, the detection limit of the sensor was 3.1 mu M (S/N = 3), with a response sensitivity of 0.16 AM-1cm-2 and reproducibility was within 6.1% in the range 1-5 x 10-3M (n = 5). Cell suspensions under normal physiological conditions had a pH between 5.5-5.7 and H2O2 concentrations in the range 7.0-20.5 muM (n=5). The addition of exogenous H2O2, as well as other potential stress stimuli, was made to the cells and the change in H2O2 concentration was monitored. This real-time quantitative H2O2 analysis is a potential marker for the evaluation of oxidative stress in plant cell cultures.