We report on the facile synthesis of highly conductive RuO2/Mn3O4 composite nanofiber mats by electrospinning of mixed ruthenium and manganese precurosr solution and their potential suitability as an active electrode in electrochemical capacitors. Electrospun RuO2/Mn3O4 fiber mats calcined at 300 degrees C exhibit multiple fiber networks composed of nanoparticles in a size range of 5-10 nm (average approximate to 7.5 nm). Partially crystalline Mn3O4 phases are well mixed with the amorphous RuOx (a-RuO2) phases while pure Mn3O4 fibers exhibit only crystalline phase. Electrochemical capacitors utilizing a-RuO2/Mn3O4 fiber mats show a high specific capacitance of 293 Fg(-1) at 10 mVs(-1), high rate capability with a capacity loss of only 45% from 10 to 2000 mVs(-1), and superior cycle performance of over 30,000 cycles as compared to those characteristics of Mn3O4 fiber mats (216 Fg(-1) at 10 mVs(-1), capacity loss of 94% at 2000 mVs(-1), poor cycle performance of 2000 cycles). These results are correlated with the improved mixed ionic/electronic conductivity of the a-RuO2/Mn3O4 fiber mats as well as to its morphological evolution, i.e., highly porous flower-like-fiber networks, during cycling tests. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3601852] All rights reserved.