The catalytic conversion of phenol in a membraneless electrochemical reactor with immobilized horseradish peroxidase (HRP) as a biological catalyst was investigated. To introduce HRP into a membraneless electrochemical reactor, we conducted immobilization on the porous Celite beads through aminopropylation with 3-aminopropyltriethoxysilane (3-APTES) and covalent linkage with glutaraldehyde (GA). Under the optimal applied potential determined by the stability of the immobilized HRP in the electric field and the production rate of hydrogen peroxide (H2O2), the aqueous phase of phenol was effectively converted by the electroenzymatic reaction. Phenol was oxidized by the electrochemical and electroenzymatic methods in a membraneless electrochemical reactor with the immobilized HRP, showing that the oxidation of phenol was enhanced by the electroenzymatic method. During the electroenzymatic degradation, phenol was converted into p-benzoquinone, various organic acids and carbon dioxide (CO2). (c) 2007 Elsevier B.V. All fights reserved.