In this work, the microstructure of NiO-CeGdO (CGO) thin films synthesized by spin coating of polymeric precursor solutions and the characteristics of the electrocatalytic reduction of NO using electrochemical cells with the NiO-(CGO) cathode interlayer were investigated. The addition of the CGO phase into NiO to form composite thin films depressed the nucleation and grain growth of the NiO main crystalline phase and decreased the impedance of the cells with the NiO-(CGO) thin films as the cathode interlayer. However, only the cells with dense NiO thin-film cathode interlayer exhibited higher selectivity for NO conversion in the presence of excess oxygen in spite of using a porous Pt cathode. The mechanism of electrocatalytic reduction of NO based on the formation of oxygen vacancies or F-center defects produced by NiO reduction in cathode interlayer are discussed. (C) 2003 The Electrochemical Society.