Experimental results of damage development in and mechanical response of heat-treated NicaloniCAS-II laminates subjected to monotonic flexure and axial loading and to cyclic tensile loading are reported. The specimens were subjected to post-processing heat treatments at 900°, 1000°, and l100°C in air for 100 hours. Changes at the fiber/matrix interface/interphase due to post-processing heat treatments were also characterized. The combined effect of fiber debonding and transverse matrix cracking in both 90° and 0° plies plays an important role in damage development in [0/90]₄₅ Nicalon/CAS-II laminates, especially in developing the secondary damage modes such as longitudinal matrix cracking and delamination. Frictional wear effects found in cyclically loaded specimens may be responsible for the observed temperature profiles during the intermediate stage of fatigue life. It is also believed that frictional wear is critical to the failure of notch sensitive fibers. Different damage modes such as "brittle" matrix crack propagation and "quasi-brittle" matrix crack propagation were observed in heat-treated specimens. Results obtained from microanalysis using an analytical scanning transmission electron microscope equipped with an energy dispersive spectrometer, and microindentation indicated that the changes of damage and failure modes were directly related to the changes of characteristics at the fiber/matreix interface/interphase.