Spinel- Ni interpenetrating network composites have been fabricated by electrodeposition of Ni in the pores of porous spinel preforms and the volume fraction of Ni in the composite is typically 50% of the pore volume of the preform. The composites have good electrical conductivity (approximately 2% that of pure Ni when the volume fraction of Ni is approximately 15%) that is controlled by the pore structure of the preform. The conductivity shows hysteresis on first heating and cooling and a slow decrease with time when held at 900 degrees C. The composites have thermal expansion closer to that of the ceramic, and higher Young's modulus and toughness than the preforms. The increases in thermal expansion and toughness are both lower than expected from composite theory; these differences are probably due to the effects of residual porosity in the composites. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1938027] All rights reserved.