In this study, acetylcholinesterase (AChE) and choline oxidase (ChO) were co-immobilized on poly (2-hydroxyethyl methacrylate) (pHEMA) membranes to construct a biosensor for the detection of anti-cholinesterase compounds. pHEMA membranes were prepared with the addition of SnCl4 to achieve the desired porosity. Immobilization of the enzymes was done by surface attachment via epichlorohydrin (Epi) and Cibacron Blue F3G-A (CB) activation. Enzyme immobilized membrane was used in the detection of anti-cholinesterase activity of aldicarb (AS), carbofuran (CF) and carbaryl (CL), as well as two mixtures, (AS + CF) and (AS + CL). The total anti-cholinesterase activity of binary pesticide mixtures was found to be lower than the sum of the individual inhibition values. (C) 2003 Elsevier B.V. All rights reserved.