A Pt electrode modified by a polypyrrole/poly(orthophenylenediamine) bilayer membrane able to entrap large molecules such as glucose oxidase was used to investigate (at 27 degrees C and pH 7) the kinetics of ascorbic acid (AA) oxidation by hydrogen peroxide (H2O2 + AA --> 2H(2)O + DAA) by following the H2O2 concentration as a function of time. The largely unmatched rejection characteristics of this device towards AA permitted it to operate even in the presence of high AA/H2O2 ratios, e.g. 1000:1. Under these conditions, pseudo-first-order kinetic constant values ranging from 3.26 x 10(-3) to 4.10 x 10(-3) s(-1) were obtained at [AA] = 2 mM and initial [H2O2] = 2 mu M. The potential influence of the above reaction on sensitivity and reliability of H2O2-detecting biosensors in the presence of AA is discussed critically, taking into account also the recent, and sometimes conflicting, literature views on the problem.