An alumina sensor using sub-micron RuO 2 sensing electrode (SE) was fabricated and examined for potentiometric dissolved oxygen (DO) detection in water at a temperature range of 9-35°C. The electromotive force (emf) response at these temperatures was linear to the logarithm of DO concentration in the range from 0.6 to 8.0 ppm (log[O 2 ], −4.71 to −3.59). RuO 2 -SE displays a Nernstian slope of −41 mV per decade at pH 8.0. It was also found that the response/recovery time to DO changes were sluggish as the water temperature cools down. Response time T 90 to DO changes increased from 8 min at a temperature of 23°C to about 30 min at a temperature of 9°C. The proton conductivity of hydrous RuO 2 appears to be due to the dissociative adsorption of water and the formation of acidic OH groups in Ru (III,IV) cluster ions. In strong alkaline solutions, the sensor's emf exhibited a mixed potential of fast and slow electrochemical reactions involving DO, RuO 4 2− and OH − ions. The results also revealed that as pH of the solution increases to pH 10.0-13.0, the response/recovery rate becomes faster, stabilizing more or less quickly depending upon the solution alkalinity. Scanning electron microscopy, energy dispersive X-rayanalysis and impedance spectroscopy techniques were used to examine respectively the morphology, crystalline structure and electrochemical behaviour of sub-micron RuO 2 oxides.