We have fabricated all solid-state thin-film microsupercapacitors (TFSCs) using RuO2 electrodes and UPON electrolytes. The RuO2 electrodes were grown at oxygen gas flow ratios [O-2/(O-2 +Ar)] of 10% and 30%. Room-temperature charge-discharge measurements show that specific capacitance is dependent on the oxygen gas flow ratio. Glancing angle x-ray diffraction (GXRD) and transmission electron microscopy (TEM) results show that the RuO2 electrodes grown at 10% contain nanocrystallites (0.7-10 nm across) embedded in the amorphous matrix, while the electrodes grown at 30% are polycrystalline (with grains of 0.7-15 nm in diameter). Based on the GXRD, TEM, Auger electron spectroscopy depth profile, and scanning electron microscopy results, the oxygen flow ratio dependence of the cycling performance of the RuO2-based TFSCs are discussed in terms of the combined effects of the microstructures of the electrodes, interfacial products, and the surface morphology of the electrodes.