Membrane materials were evaluated for use in amperometric biosensors. PVC, polysulfone (PS) and sulfonated polyether-ether sulfone–polyether sulfone (SPEES-PES) and SPEES-PES/PVC composites were solvent cast. Varying the plasticizer (isopropyl myristate) content of PS and PVC membranes enabled electrode response to hydrogen peroxide and acetaminophen to be modulated. Omission of plasticizer from PS and PVC resulted in hydrogen peroxide permeable membranes, selective against acetaminophen, ascorbate and urate (ratios 230 : 1 in each case). SPEES–PES copolymer membranes (unplasticized) showed greater hydrogen peroxide permeability (corresponding ratios 1430 : 1). By mixing plasticized PVC and SPEES–PES in suitable proportions, permeability ratios of hydrogen peroxide : acetaminophen were controlled. Glucose sensors using these composite membranes for H₂O₂ perm-selectivity demonstrated linear responses through the clinical range, and ⩽5% loss of signal on exposure to blood. A novel form of glucose oxidase based biosensor deployed a new membrane as an outer layer selective for the glucose substrate, rejecting interferents. Accordingly, a second selective layer for the enzyme reaction product (hydrogen peroxide) is not required. The new membrane is a plasticized mix of PVC and SPEES–PES polymers incorporating isopropyl myristate. Selectivity is adequate for clinical glucose measurement, and in particular acetaminophen interference is excluded in addition to ascorbate and urate, representing an advance over membranes described hitherto. A linear range of 0–20 mmol l^–1 glucose confirms control of substrate diffusion sufficient to extend apparent enzyme K_M, and resistance to fouling in blood is also promising for deployment in a clinical assay.