Sol-gel technique has been used for the covalent immobilization of the water-soluble mediator, thionin to construct a bulk modified, leak free composite electrode. This renewable composite electrode provides stable immobilization matrix for thionin via glutaraldehyde crosslinking. In the electrode composition the sol-gel precursor 3-aminopropyltrimethoxy silane serves as the host for immobilization of thionin, thereby preventing its leakage. An additional precursor methyl trimethoxy silane endows hydrophobicity and limits the wetting section of the modified electrode. Cyclic voltammetric characterization of the modified electrode in the potential range of 0.2 to -0.6 V exhibited stable redox peaks with a formal potential of -0.273 V, corresponding to immobilized thionin. This chemically modified electrode exhibits good electrocatalytic activity for the reduction of H(2)O(2) at a lower potential of -0.35 V. The reduction current of the modified electrode increases linearly in the range of 3.44 x 10(-6)M to 3.07 x 10(-3)M H(2)O(2) with a detection limit of 1.38 x 10(-6)M. The stable and quick response (5s) during chronoamperometry shows the potential application of the modified electrode for flow system analysis. The low potential operation (-0.35 V) favoured selective determination of H(2)O(2). The composite electrode exhibits distinct advantages of polishing in the event of surface fouling as well as simple preparation, good chemical and mechanical stability, economical and remarkable long-term stability (more than 1 year). The applicability of the present sensor for H(2)O(2) determination proposes a method for the detection of other biologically significant analytes.