RuO2-glass composites, whose electrical conductivity can be tuned by changing the phase ratio, provide the basis for thick-film resistors (TFR), which are widely used in technology, but they also simulate an ultimate glass nuclear waste form. Despite their technical importance, the conduction mechanisms are not yet understood, especially at molten state. This paper reports electrical investigations of RuO2-glass composites from molten to glassy states, using the four-electrode impedance measurement method. The electrical conductivity of RuO2-glass composite reveals that, even in the case of low RuO2 content, electronic and ionic transports occur over a wide temperature range. The RuO2 dispersion significantly enhances the total electrical conductivity up to 1300 degreesC. Direct current (DC) measurements demonstrate that both ion and electron transport occur at the molten state. (C) 2004 Elsevier B.V. All rights reserved.