An electrode system is disclosed for measuring a concentration or presence of an analyte under in-vivo conditions, where the electrode system includes at least one electrode with immobilized enzyme molecules and an improved diffusion barrier that controls diffusion of the analyte from body fluid surrounding the electrode system to the enzyme molecules. The diffusion barrier includes a hydrophilic polyurethane or a block copolymer having at least one hydrophilic block and at least one hydrophobic block. The electrode system also can include a spacer membrane that includes a hydrophilic copolymer of acrylic and/or methacrylic monomers.
