An electrochemical microsensor has been developed for the simultaneous assay of ascorbate (AA) and urate (UA) antioxidants in human blood serum. The electrode surface was modified by means of electropolymerized conductive poly(3,4-ethylenedioxythiophene) PEDOT organic films. Highly selective responses (detection potential difference more than 250 mV) were obtained for both biomarkers by optimizing the PEDOT thickness. Contrary to most previous studies the analytical performances were recorded by testing the microsensor directly in blood serum without any pretreatment of the samples. Using differential pulse voltammetry (DPV) the sensitivity and detection limit were 0.481 µA µM-1 cm-2 and 4.2 µmol L-1 for AA and 1.815 µA µM-1 cm-2 and 2.0 µmol L-1 for UA. The calibration curves were linear in the concentration range 5–200 µmol L-1 for AA and 3–700 µmol L-1 for UA. The accuracy of the sensor was satisfactorily compared with the reference analytical methods provided that the synergic effect between both antioxidants was considered. The sensor response was unmodified in the presence of the major biochemical interfering species.