This paper describes a simple, fast and sensitive method for the determination of hydrogen peroxide in liquid samples. The system, consisting of a mini-reactor containing horseradish peroxidase (HRP) immobilised onto polyaniline coupled to a spectrophotometer, was able to determine hydrogen peroxide in the range from 55 to 2730 nmol L-1 using pyrogallol as the reduced substrate. The determinations were made using a flow injection system with a flow rate of 3.0 mL min(-1) and optical detection at 420 nm with a glass flow cell having a I mm optical path length. The system has good response repeatability (R.S.D. 0.3-2.3%), while the mini-reactor can be stored and reused with up to 1500 injections without significant reduction of enzyme activity. (c) 2005 Elsevier Ltd. All rights reserved.