Ruthenium oxide (RuO2) is commonly used in resistive pastes for screen printing. The electrochemical properties of a screen-printing planar RuO2 electrode have hardly been studied. In this communication, planar electrochemical sensors with a RuO2 working electrode, an AglAgCl reference electrode and a RuO2 counter electrode are fabricated by screen printing. The electro-oxidation of ascorbic acid, uric acid, and hydrogen peroxide on these RuO2 electrodes is investigated by means of cyclic voltammetry. Compared to uric acid and hydrogen peroxide, ascorbic acid can be easily oxidized at the low operating potential ( < 150 mV versus AglAgCl). The amperometric measurement of ascorbic acid at 100 mV on a RuO2 electrode shows fast response and good linearity in the 0-4 mM range. Meanwhile, the electrochemical interference from uric acid and hydrogen peroxide at this potential is very small. (C) 2000 Elsevier Science S.A. All rights reserved.