AbstractA new enzymatic amperometric sensor for hydrogen peroxide based on a modified carbon paste electrode is described. Horseradish peroxidase and ferrocene mediator were immobilized by dissolution or dispersion in a graphite–paraffin oil paste. To prevent losses of immobilized chemicals under operational conditions, the electrode surface was coated with a Nafion membrane. The device exhibits a relatively fast response time (42 s for 95% of the steady‐state response) and yields currents linearly related to the hydrogen peroxide concentration over the 10−7to 10−5mol/L range in both static and flow injection operations. The slope of the calibration plot remained constant for more than 20 days. Flow injection assays of hydrogen peroxide at a sampling rate of about 70 injections per hour, with a relative standard deviation of 1.4%, are possible. An easy and low cost preparation procedure is de