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Both thermal and mechanical durabilities of porous cordierite ceramic monoliths depend on their strength and deformation behavior over the operational temperature range. In this paper we present the modulus of rupture data for EX-20, 400/6.8 square cell substrates of cordierite composition over the temperature range 70° - 2550°F. These data show a monotonic increase in four-point MOR up to 2200°F where its value approaches twice that at 70°F. The deformation mode is elastic and culminates in brittle fracture up to this temperature. Above 2200°F, however, the strength begins to decrease and the deformation mode becomes ductile; the brittle fracture is not observed. Instead, the surface crack propagates partially and is soon arrested. The failed specimen remains intact and shows permanent curvature. The transition from brittle to ductile behavior and the mechanism responsible for it will be discussed.