The successful use of biosensors requires that the sensor can operate over abroad enough linear range that encompasses the physiological concentration of the substrate of interest. A polymer coating layer functioning as a mass-transport barrier is typically used to expand the linear range of biosensors with, however, the concomitant disadvantage of a reduction in the response. Effects of a poly(acrylic acid) coating layer on the response of a glassy carbon electrode modified with an Os redox polymer and lactate oxidase were evaluated. The coating layer resulted in an expanded linear range from 7 to 15 mM, doubled catalytic response towards the oxidation of 35 mM lactate, and improved operational stability. Detailed voltammetry studies revealed that the coating layer can improve the amount of the redox polymer that is available as a mediator, leading to the increased catalytic response at high concentrations of substrate. Similar results were obtained with other polymer layers [polystyrene sulfonate,poly(diallyldimethyl-ammonium chloride), and poly(3,4-ethylenedioxythiophene)] and with the enzymes glucose oxidase and bilirubin oxidase, demonstrating the general nature of the method.