The University of Dayton is investigating the effects of environment on the mechanical behavior of commercially available structural ceramics being considered for automotive engine applications.* In Phase III of this investigation, six Si3N4 and three SiC materials were evaluated from 20°C to 1400°C in air. Flexural strength and flexural dynamic fatigue behavior were determined for all materials studied. Tensile strength and static, dynamic, and cyclic tensile fatigue behavior were determined for two Si3N4 and one SiC. In addition, chemistry, microstructure, and other physical properties were investigated.