Nickel cobalt (Ni-Co) oxides with various Ni/Co ratios are synthesized by using a sol-gel process. Electrochemical performance, microstructure, morphology and BET surface area are found strongly related to Ni and Co concentration. Increasing Co concentration changes the oxide microstructure from NiO crystal dominate structure (Ni: Co 1:1 and 1:2) to Co3O4 dominate structure (Ni:Co 1:4 and 0:1). A maximum specific capacitance of 1539 Fg(-1) was obtained for Ni-Co (Ni: Co 1:2) oxide at a current density of 1Ag(-1), and this capacitance is similar to that of RuO2. A systematic study shows that the Ni-Co oxide (1:2) has a mesoporous structure with a high BET surface area of similar to 315 m(2) g(-1) and porous size of similar to 4.7 nm, which is favorable for the charge/discharge process of a supercapacitor. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3574021]