An electroactive composite film (MWCNT-NiOx) containing multi-walled carbon nanotubes (MWCNTs) with nickel oxide (NiOx) was synthesized on glassy carbon electrode, gold and indium tin oxide (ITO) by cyclic voltammetry (CV). The surface morphology of the composite films deposited on transparent semiconductor ITO was studied using scanning electron microscopy and atomic force microscopy. The quantity and the rate of NiOx deposition on bare and MWCNT modified electrodes were studied using electrochemical quartz crystal microbalance (EQCM). The EQCM results showed that MWCNT catalyses NiOx electrodeposition. Further, the CV studies also proved the presence of MWCNTs in the composite film enhanced the surface coverage concentration of NiOx. The MWCNT-NiOx film showed characteristic voltammetric peaks of NiII/III redox couple. However, the reversibility, i.e., the electrocatalytic activity of NiII/III entirely depends on pH of the electrolyte. The optimized pH range for good electrocatalytic activity was found to be pH 9 to 13. MWCNT-NiOx composite film can catalyze the oxidation of water and hydrogen peroxide with the evolution of oxygen, where the evolved oxygen is detected using the same composite film. Therefore, the determination of H2O and H2O2 oxidation has been done indirectly by measuring the evolved oxygen at MWCNT-NiOx by CV.