The microstructure and structural evolution in the composite NiO-YSZ electrocatalytic electrode during operation of electrochemical cells for NO decomposition has been observed and investigated. The structure of the electrocatalytic electrode was found to contain two types of pores. The first group includes pores with a size range of 10-100 nm forming chains or bands. They may form channels by overlapping. These pores are located in YSZ grains and they remain unchanged during cell operation. The second group includes smaller pores located in NiO grains at near-boundary regions with YSZ grains or directly in the NiO-YSZ interface. These regions are unstable during the cell operation and new Ni and NiO grains nucleate and grow in the interfacial areas. The formation of such new grains as a result of the electrochemical processes in the cell has been discussed. The correlation between structure and properties has been studied.