New ceramic gas separators for solid oxide fuel cells (SOFCs) were fabricated with a material of 42 mass % (MA) as a base material. Thermal and bonding properties of the material were examined. The material of the MA has excellent thermal expansion compatibility with the yttria-stabilized zirconia electrolyte between 100° and 1400°C. The thermal conductivity of the MA at 25 and 1000°C was 16 and 6 Wm−1 K−1, respectively. These values were higher than those of the The bonding between the MA blocks was conducted under the condition of 1200°C and 12 kPa with a glass as a bonding agent. According to scanning electron microscopy and electron probe microanalysis, the bonded interface was dense enough to prevent gases from penetrating. Five-cell planar SOFC stacks were fabricated with 120 cm2 cells and the developed separators. Connecting the three stacks electrically in series enabled module to be made where a power generation test was carried out at 1000°C with hydrogen gas and air. The developed SOFC module recorded the almost theoretical open-circuit voltage of 15.9 V. It attained the maximum output power of 294 W at a current of 40.4 A and a fuel utilization of 78.6%. © 2003 The Electrochemical Society. All rights reserved.