This article presents a bioelectrochemical system for the determination of pesticides by enzyme inhibition data. Measurements are performed by electrochemically monitoring the inhibition of the catalytic activity of the enzyme alkaline phosphatase (ALP) either in the presence or in the absence of pesticides, and particularly of Malathion and of 2,4-dichlorophenoxyacetic acid (2,4-D), representatives of the general classes of organophosphorous and organochlorinated agents. ALP catalytic activity was determined by using two different analytical configurations: (a) an amperometric ALP based biosensors using 3-indoxyl phosphate as the enzyme substrate; (b) a system allowing the voltammetric determination of the electroactive products of the ALP catalyzed reactions by using two different substrates: phenyl phosphate (PP) and ascorbate-2-phosphate (A-2-P). Studies of cyclic voltammetry and amperometry were performed first to define the optimal experimental conditions of the electrochemical measurements. For the optimal experimental conditions (Tris-0.1 M HCl aqueous buffer solution at pH 8.0, incubation time of 30-60 min) a detection limit of 0.5-6 mug 1(-1) is obtained, with a response, which is linear over 1-2 decades of concentration. (C) 2004 Elsevier B.V. All rights reserved.