The response of glucose oxidase (GOx) modified poly(o-phenylenediamine) coated Pt disk electrodes to glucose was well-behaved with a rapid response time and displaying Michaelis-Menten kinetics. However, the glucose response was lowered in a concentration-dependent manner by ascorbic acid when the glucose calibrations were carried out in solutions containing this reducing agent. The possibility of a homogeneous redox reaction in which the H2O2 generated by tie enzymatic oxidation of glucose at the GOx/polymer surface is consumed by ascorbate was investigated. Similar ''negative'' interference at GOx-modified carbon powder electrodes not involving membranes and for H2O2 calibrations at bare Pt electrodes supported the hypothesis. The observation that this interference could be blocked by the chelating agent EDTA suggests that the homogeneous reaction is catalyzed by trace metal ion impurities in solution. A model for the homogeneous reaction based on these experimental findings is proposed and tested by comparing quiescent and stirred solutions. No homogeneous interference by uric acid was observed. The electrodes were found to be free from lipid fouling in vitro, and experiments monitoring brain glucose levels in vivo indicate the absence of the homogeneous reaction in this environment. The results highlight the need to test each individual assay procedure involving H2O2 under relevant conditions for both positive and negative interference by ascorbic acid.