The membrane potential and charge density of membranes based on acrylonitrile copolymer were studied before and after immobilization of glucose oxidase. Membranes surface modified with sodium hydroxide, hydroxylamine, hydrazine hydrate, dimethylaminoethyl methacrylate and 2-acrylamido-2-methylpropanesulfonic acid were used, as well as membranes prepared from acrylonitrile copolymer modified in bulk with sodium hydroxide and hydroxylamine. The membrane potential was found to be proportional to the amount of the ionogenic groups in the modified membranes. The effects of electrolyte type, concentration and pH of the NaCl solution on the membrane potential were studied. The charge densities of modified membranes before and after immobilization of glucose oxidase were calculated. A correlation between the amount of bound protein and membrane potential was found. The influence of substrate-glucose concentration on membrane potential of enzyme bound membranes was studied.