The thermal expansion of NiO-8 mol % Y2O3-stabilized ZrO2 (YSZ) composites and Ni-YSZ cermets in air and hydrogen have been investigated in the temperature range from 50 to 1000 degrees C. The average linear thermal expansion coefficient (TEC) of NiO-YSZ composites in air increased with NiO content over the entire composition range. While NiO in the composites was changed to Ni in the H-2 stream, their expansions were governed by the reduction of NiO. For reduced, Ni-YSZ cermets, the TEC increases significantly with Ni content in the composition range >60 vol % Ni. The TEC increased gradually during repeated thermal cycles between room temperature and 1000 degrees C. When cermets were measured in air, the Ni particles were fully oxidized to NiO above 900 degrees C, and many cracks appeared in the samples.