Nickel oxides on carbon nanotube electrodes (NiO x /CNT electrodes) are prepared by depositing Ni(OH)2 electrochemically onto carbon nanotube (CNT) film substrates with subsequent heating to 300°C. Compared with the as deposited Ni(OH)2 on CNT film substrates (Ni(OH)2/CNT electrodes), the 300°C heat treated electrode shows much high rate capability, which makes it suitable as an electrode in supercapacitor applications. X-ray photoelectron spectroscopy shows that the pseudocapacitance of the NiO x /CNT electrodes in a 1M KOH solution originates from redox reactions of NiO x /NiO x OH and Ni(OH)2/NiOOH. The 8.9wt.% NiO x in the NiO x /CNT electrode shows a NiO x -normalized specific capacitance of 1701Fg−1 with excellent high rate capability due to the 3-dimensional nanoporous network structure with an extremely thin NiO x layer on the CNT film substrate. On the other hand, the 36.6wt.% NiO x /CNT electrode has a maximum geometric and volumetric capacitance of 127mFcm−2 and 254Fcc−1, respectively, with a specific capacitance of 671Fg−1, which is much lower than that of the 8.9% NiO x electrode. This decrease in specific capacitance of the high wt.% NiO x /CNT electrodes can be attributed to the dead volume of the oxides, high equivalent series resistance for a heavier deposit, and the ineffective ionic transportation caused by the destruction of the 3-dimensional network structure. Deconvolution analysis of the cyclic voltammograms reveals that the rate capability of the NiO x /CNT electrodes is adversely affected by the redox reaction of Ni(OH)2, while the adverse effects from the reaction of NiO x is insignificant.