A polyion complex membrane containing physically entrapped microperoxidase and ferrocene was prepared on a glassy carbon electrode. A reductive current response for H2O2 was obtained under the application of 0.0 V versus Ag \ AgCl. The current response was proportional to the H2O2 concentration up to 20 mu M, and the lower detection limit was 0.5 mu M (S/N = 5). The interference current from L-ascorbate was ca. 45% of the current for the same concentration of H2O2. The electrode could be used for 10 days. The electrode was applied as the transducer of an oxidase-based sensor such as a glucose oxidase-based one: the glucose-sensing electrode prepared showed high sensitivity (lower detection limit, 5 mu M).