A colorimetric detection system based on horseradish peroxidase (HRP) immobilized microporous Poly (styrene-divinylbenzene-polyglutaraldehyde) monolith was developed for phenol biosensing. The incorporation of the aldehyde groups to the backbone of the microporous (90%) monolith was achieved by the polymerization of the continuous phase of a high internal phase emulsion. HRP was chemically immobilized onto the inner walls of the micropores of the monolith via the aldehyde groups. HRP immobilized microporous monolith (HRP-MM) was used as a biocatalytic cell under forced flow conditions in a flow injection system. A carrier stream containing 4-aminoantipyrine (4-AAP) as color reagent was flowed through the HRP-MM at various flow rates ranged between 0.1-15 mL min(-1). The analytical parameters of response time, detection limit and linear range varied at the range of 1-60 s, 1-250 mu M, and 0.25-2 mM, respectively. The results showed that the most accurate phenol biosensing was obtained at the flow rate of 1 mL min(-1). The microarchitected structure of the monolith improved the mass transferring conditions for the enzymatic reaction due to the forced flow through the micropores. The detection system presented a satisfactory precision evaluated by relative standard deviation of 1.1% (n = 13) and recovery of 100% at the flow rate of 1 mL min(-1). The system showed 60% of its initial signal from 12th day up to the two months. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.