The electrical transport and the microstructure of RuO2-glass composites with metal oxide volume fractions 0.01 less than or equal to f(RuO2) less than or equal to 0.4 have been investigated by SEM, HREM and frequency and electric field dependent conductivity studies at 4-300 K. Different transport mechanisms controlled by the microstructure of the composite have been identified. In the limiting cases of high and low RuO2 contents metallic or ionic transport prevails. For intermediate concentrations, 0.05 less than or equal to f(RuO2) less than or equal to 0.2, hopping and tunneling transport are superposed. Hopping transport has been identified by frequency dependent conductivity studies; evidence for tunneling transport stems from the electric field dependence of the conductivity at temperatures < 100 K. The significance of the latter transport mechanisms rests on the densely packed microstructure of these composites. Under the preparation conditions involved (1100 K/15 min), predominantly RuO2 clusters with a mean size of 250 nm and comparatively short intercluster distances form within the glass matrix. As importantly, each of these clusters consists of a large number of ultrafine RuO2 particles (n > 10(3)) separated by a uniform glass layer of less than or equal to 2 nm thickness, i.e., the metal oxide particles remain suspended in the matrix. Based on the statistical evaluation of the SEM- and HREM-data a quantitative description of the electrical transport behaviour of RuO2-glass composites is presented.