The electrochemical behavior of mixed Ru–Sn oxides fabricated by a modified sol–gel process and a co-annealing method are systematically compared (using cyclic voltammetric and electrochemical impedance spectroscopic analyses), for the application of supercapacitors. Influences of annealing temperature and time on the specific capacitance and textural properties of oxides are also studied. Based on the quasi-linear dependence of voltammetric current density on the scan rate of CV (from 25 to 500mVs−1), the lowest proton diffusion resistance, and the high-frequency (∼426Hz) capacitive responses of the co-annealed composite (RTOCco) as well as its ease in composition control, co-annealing of hydrous RuO2 and SnO2 at 200°C for 2h is recommended as a promising and practical method for preparing mixed oxides in the supercapacitor application.